This morning I printed out a new copy of the Cryptic as the other one was a mess! Filled in letters neatly, plus a few more answers, but still have quite a few more to do. 4 down is a bit of a puzzle. Clue is:
Boss, hard-working, havin debts cancelled (4)
Letters: - T - D
The only two letters to add which make a word appear to be STUD, but I can't see a connection.
I think the T and D are correct. The T is the last letter of 7 across:
Mostly grumpy character in bar (

CROTCHET
and the D is the first letter from 11 across:
Explosive made tiny fragments (

DYNAMITE
What do you think?