Can a Druid/ War Cleric Multiclass use War Priest to make a bonus attack?

Question

The 5e PHB states on page 63 about War Priest:

When you use the attack action, you can make one weapon attack as a bonus action.

So this rules out any beasts making a bonus attack after a multiattack, but what if an Awakened Tree for example uses his action to make a Slam Melee Weapon Attack. Can the player then make a second slam attack as a bonus attack?

I believe this would qualify and the beast's weapon attack would be considered as a valid triggering attack to create a bonus attack. The MM pp.10-11 says

The most common actions that a monster will take in combat are melee and ranged attacks. These can be spell attacks or weapon attacks where "weapon" might be a manufactured item or a natural weapon such as a claw or tail spike (emphasis mine)

What do you think? Can a Druid/ War Cleric Multiclass use the War Priest ability to make a bonus attack while wildshaped?

Answer 1

Yes, you can use War Priest but not if you use Multiattack

Taking a cue from András's answer, the attacks listed in a creature's stat block are not Actions but attack modes or melee attacks. Here's what I think you're missing:

Page 192 of the PHB defines the Attack Action:

Attack

With this action, you make one melee or ranged attack.

The PHB further defines what a melee attack is and, in page 195, it explicitly states that:

Melee Attacks

A typical monster makes a melee attack when it strikes with its claws, horns, teeth, tentacles, or other body part.

With the above reading, the PHB confirms that Claw, Gore, Bite, and Slam are melee attack options that can be chosen when a creature uses the Attack Action and therefore able to trigger War Priest.

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But this is where it gets weird:

Multiattack is a distinct Action that details what attack options are used by the creature when the creature uses Multiattack (not the Attack Action!). Since it is a separate Action (like Ink Cloud and Swallow) it cannot trigger War Priest.

This means creatures using Multiattack cannot trigger War Priest; they must use the regular Attack action and then use a Bonus Action to attack with War Priest, but this obviates the need to use War Priest as the Druid/Cleric can already attack twice with Multiattack.

As pointed out by KorvinStarmast, a beast with different attack options in the Multiattack Action like the Brown Bear could potentially get a slight damage increase if the Druid chooses not to use Multiattack (Claw [2d6+4] + Bite [1d8+4]) but instead use War Priest to attack twice with Claw (4d6+8). This allows for a unique dynamic that a Druid/Cleric can use during combat, when needed.

Answer 2

You can use Slam as a bonus action

The rules are quite a bit confusing, mostly because the creators of Monster Manual did not consider the Druid, much less its many multiclassing options.

Seemingly Christopher's answer is right, but if it really were, most monsters could not use Opportunity Attacks, as they usually have only Actions listed.
The only logical conclusion is that the Actions listed are actually attack modes, usable in every way a Weapon Attack would be, like normal Action, Opportunity Attack, or even with Extra Attack. And as Bonus Actions.

Answer 3

Slam is its Own Action

When a monster takes its action, it can choose from the options in the Actions section of its stat block or use one of the actions available to all creatures, such as the Dash or Hide action, as described in the Player's Handbook.

–MM, page 10.

The war priest's feature triggers when you take the attack action, not the slam action—they are two different actions.

Of course, there is nothing stopping you from taking the attack action instead, but the lack of any weapons makes this a poor option.