If you use a Clockwork Amulet on an attack where you have advantage or disadvantage, do you not make a roll at all, or does it just give a "10" on one of the two dice you will roll?
Relevant Clockwork Amulet text:
When you make an attack roll while wearing the amulet, you can forgo rolling the d20 to get a 10 on the die. Once used, this property can't be used again until the next dawn.
-- Xanathar's Guide to Everything
To start with, let's look at the relevant text of the amulet's description:
When you make an attack roll while wearing the amulet, you can forgo rolling the d20 to get a 10 on the die.
A naive reading of that would hold that advantage and disadvantage are irrelevant: you "forgo rolling the d20" and "get a 10 on the die".
As a reminder: "To make an attack roll, roll a d20 and add the appropriate modifiers. If the total of the roll plus modifiers equals or exceeds the target's Armor Class (AC), the attack hits". Which is to say that, ultimately, we care about the value of the total of "the roll".
So: it's clear that, absent advantage or disadvantage, someone using a clockwork amulet in conjunction with an attack will proceed as if they'd rolled a 10 on the die. I argue that it's equally correct that the attack proceeds as if they'd rolled a 10 even if they have advantage or disadvantage.
Advantage and Disadvantage are defined as:
Sometimes a special ability or spell tells you that you have advantage or disadvantage on an ability check, a saving throw, or an attack roll. When that happens, you roll a second d20 when you make the roll. Use the higher of the two rolls if you have advantage, and use the lower roll if you have disadvantage. For example, if you have disadvantage and roll a 17 and a 5, you use the 5. If you instead have advantage and roll those numbers, you use the 17.
Fundamentally, advantage and disadvantage alter the "roll a d20" part of an attack. Both insert a second die and force the player to use one of the two - that is, the higher (for advantage) or lower (for disadvantage) result becomes the die that is used to determine whether the attack meets the target's AC.
The clockwork amulet bypasses actually rolling a die entirely, instead ensuring that the attack is treated as if a natural 10 had been rolled. It doesn't matter whether that 10 is "rolled" on a single die, with advantage, or with disadvantage: the attack proceeds as if the d20 used for the attack came up 10.
Yes, they do. The amulet doesn't remove advantage and disadvantage, it simply forces the result of "the roll". The attack is still being made with advantage or disadvantage, which may still have mechanical effects (eg., a rogue would still get sneak attack because they "have advantage on the attack roll").
No, I don't believe it is. The amulet is only usable once a day, which means that it can only affect one attack out of, IME, 3-5 encounters or 9-25 rounds of combat (both very rough numbers, of course; YMMV). Over the course of that many rounds, being able to guarantee a roll slightly under average (remember: average on a d20 is 10.5) on a single attack is, at best, a nudge in the PC's favor - and only when they'd hit on a 10, at that*. It may help a little in mitigating the effective attack penalty of disadvantage, but that help still requires that a 10 on the die hits the target; that's not uncommon, but it's by no means a guarantee.
* Or if using a fumble mechanic where a nat-1 on an attack roll causes something worse than a guaranteed miss, etc.. I've played - briefly - at tables where a nat-1 could lead to the PC dropping their weapon or hitting an ally instead of their intended target; at those tables, this is a marginally more effective boon, but only just.
The Clockwork Amulet states (emphases mine):
When you make an attack roll while wearing the amulet, you can forgo rolling the d20 to get a 10 on the die.
It is clear here that the amulet can affect one die only, regardless of how many attack dice are involved in the actual attack.
The PHB description of advantage says:
Sometimes a special ability or spell tells you that you have advantage or disadvantage on an ability check, a saving throw, or an attack roll. When that happens, you roll a second d20 when you make the roll. Use the higher of the two rolls if you have advantage, and use the lower roll if you have disadvantage.
From here it is clear that advantage turns what would be one die roll into two rolls, and that both of these dice are rolled at the same time ('when you make the roll') and not one after the other. After this, the advantage or disadvantage selects from these to decide which of the two rolls will manifest as the roll actually used.
Putting how the Amulet works together with advantage, the Clockwork Amulet replaces one die roll, but it does so before the roll itself is made ('you can forgo rolling'). When your DM says that you have advantage on a roll, you could announce that you were using the Amulet, but only before you actually rolled. You would then forgo rolling one die and still roll another. Your two results would be a 10 and whatever the natural 'second' roll was. Advantage would then select the higher of these two 'rolls' to become the realized attack roll.
Note that minmass's accepted answer claims that since the amulet can replace only one roll, it replaces the final, realized roll that would have resulted from advantage or disadvantage. To do so, it has to argue that the two initial rolls made for the attack are not really attack rolls, only the final one is. That doesn't sit right with me, since in any other context both of these rolls would be considered attack rolls.
For example, the halfling's lucky racial feature says "When you roll a 1 on the d20 for an attack roll, ability check, or saving throw, you can reroll the die and must use the new roll." Suppose a halfling had advantage and rolled an 8 and a 1. By minmass's definition, the halfling would not be permitted to reroll the 1, since only the 8 counted as the actual attack roll. This does not seem like the intent of the ability.
To rule this way also shuts down the explicit direction of advantage, "When that happens, you roll a second d20 when you make the roll." What if the attacker also had elven accuracy? Or the Lucky feat (which works differently from the Halfling's Lucky and can give an additional attack die)? Ruling that the Amulet has the power to shut down each of these features when it doesn't explicitly say it does, seems unjustified to me.
The amulet says:
When you make an attack roll while wearing the amulet, you can forgo rolling the d20 to get a 10 on the die.
As written, this assumes you only roll a single die. It does not say "You can forgo rolling a d20 to get a 10 on the roll". So it does not apply to attack rolls where you roll multiple d20.
When you attack with advantage or disadvantage you are rolling two d20, not one, so it does not apply.
Does that make the amulet less powerful? Yes. Making the amulet a little less powerful is in line with how common magic items from Xanathar are introduced:
These items seldom increase a character's power, but they are likely to amuse players and provide fun roleplaying opportunities.
Among those items, even if it only works on normal attacks, this is already one that does increase the character's power, and is leaning into mechanics more than many others. The amulet also does not require attunement, so for min-maxers this is just pure gravy on one of the attacks they make per day. Compare this to stuff like a cloak of billowing, that you can let billow dramatically, or the dread helm that lets your eyes glow red. Those are mostly for style and roleplaying opportunities.
When you know an opponent can be hit on a 10, then the amulet removes the risk of missing entirely, which can massively increase your hit chance. The average chance to hit is 65%, so you would miss on a roll of 7 or worse. Across tiers, against an average AC opponent, this translates to an auto-hit. This is already a powerful effect. If it would work with disadvantage, that would be another big swing by reducing the effect of disadvantage to partially blank it.
But power arguments are only a secondary consideration, I think it does not work due to the wording, not because it would be too powerful. Because the amulet is limited to a single use per day, I don't think it would be unbalancing to allow it replacing all the rolls or one of the rolls of an attack with advantage or disadvantage, if a DM desired so. But that's not how it's written.
As you can see from there now being at least 4 different answers to your question, including this one, within less than 24h, there doesn't seem to be a consensus on the RAW interpretation of the amulet interacting with advantage and disadvantage, at least not yet. Instead of trying to muddle along here myself, I would like to give you an alternative solution, that might help you and your group handle this. This is most likely not RAW, however it is based on a different set of rules from the PHB, which we could apply here.
The Clockwork Amulet allows you to automatically roll a 10 on the D20 instead of rolling the die manually. There is a different concept in-game that does the exact same thing: passive checks. When your DM wants you to roll for something without rolling, they can make your character do a passive check. Your "roll" for such a check is 10 + your bonus to the roll, exactly as it is with attacking using the Clockwork Amulet.
Now how do passive checks handle advantage and disadvantage? The PHB explains on page 175:
If the character has advantage on the check, add 5. For disadvantage subtract 5.
You can use the same rule to handle advantage and disadvantage when using the Clockwork Amulet at your table. Again this might not be the RAW solution, so your group would have to house rule this, but this is probably going to be a really simple and intuitive solution and even though it might not be rooted in the rules specifically for this item, it is still rooted in the game's rules at large.
