Numeric A-Z crossword

Question

This is an A-Z crossword: each of the 26 answers begins with a different letter.

? $ 10^{3 + 3 \cdot 10^{ \ 3 \cdot 10^{ \ \ 3 \cdot 10^{ \ \ 21}}}} $

Clues:
0 Fearless battleship (6)
1 Or eleven (3)
2 Sibling (4)
3 All for one... (10)
4 Or ghetto (7)
5 Chiseled tenon gap (8)
6 Easily engage gaggle (12)
7 Turned sawed key (8)
8 Avocet's harmony (6)
9 Obscured by veils (5)
10 E.g. noughties (6)
11 WYSIWYG (6)
12 Heavens above (6)
13 For some (7)
14 Unwrapped north gift (9)
15 Your big nun is game (10)
18 Of course (5)
19 Palindromic (3)
20 Chaos ironed out (11)
21 Not bridge – pontoon (9)
22 Circular-logic (5)
24 Pied bald bricks (10)
36 Ale measure (4)
39 Pests in thriller (5)
1000 Klueless (4)
? Jolly big number (9)

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Edit: I ran the puzzle past a friend before posting it, just as you see it but without the list of 26 clues. They said

Impossible! There aren't any clues.

I replied

There are! Look at the title. But ok, here is the rest of each clue.

Answer 1

The solved crossword looks like this:

The trick here is (as per the title)...

...that the clue numbers are a vital part of the clues, pointing towards the answer. The words beside them give a further indication, often in the form of an anagram.

The clue explanations are as follows:

0 Fearless battleship (6) NOUGHT - from (-dread)NOUGHT;
1 Or eleven (3) ACE - a card worth either 1 or 11 in blackjack;
2 Sibling (4) TWIN - one of two siblings;
3 All for one... (10) MUSKETEERS - of which there are three in the famous work, with their famous motto: "All for one, and one for all!";
4 Or ghetto (7) QUARTER - $1/4$ or a synonym for a part of town with a particular culture or character;
5 Chiseled tenon gap (8) PENTAGON - a five-sided shape, anagram of TENONGAP;
6 Easily engage gaggle (12) GEESE A-LAYING - six are given in The Twelve Days of Christmas, anagram of EASILYENGAGE;
7 Turned sawed key (8) WEEKDAYS - seven in a week, anagram of SAWEDKEY;
8 Avocet's harmony (6) OCTAVE - a series of eight notes, anagram of AVOCET;
9 Obscured by veils (5) LIVES - a cat has nine, anagram of VEILS;
10 E.g. noughties (6) DECADE - a period of ten consecutive years, e.g. the 2000s;
11 WYSIWYG (6) ELEVEN - 'what you see is what you get', a straightforward clue here;
12 Heavens above (6) ZODIAC - 12 astrological signs;
13 For some (7) UNLUCKY ("13 - unlucky for some");
14 Unwrapped north gift (9) FORTNIGHT - 14 days, anagram of NORTHGIFT;
15 Your big nun is game (10) RUGBY UNION - a sport involving teams of 15 players, anagram of YOURBIGNUN;
18 Of course (5) HOLES - 18 of them on a golf course;
19 Palindromic (3) XIX - a palindromic representation of the number 19 (in Roman numerals);
20 Chaos ironed out (11) ICOSAHEDRON - a convex polyhedron with 20 faces, anagram of CHAOSIRONED;
21 Not bridge – pontoon (9) VINGT-ET-UN - a card game, also known as '21', 'pontoon' or 'blackjack';
22 Circular-logic (5) CATCH - as in a "Catch-22 situation";
24 Pied bald bricks (10) BLACKBIRDS - "four and twenty blackbirds baked in a pie", anagram of BALDBRICKS;
36 Ale measure (4) YARD - 'a yard of ale', with a yard being 36 inches;
39 Pests in thriller (5) STEPS - The 39 Steps, an anagram of PESTS;
1000 Klueless (4) KILO - metrix prefix for 1000 (and an extra hint in the leading 'K' of 'K'lueless as to which letter to focus on);
? Jolly big number (9) JOVILLION, with 'jolly' connecting to the sound of 'jovial' also.