Firm answer man accepts (7)
I believe the answer is:
earnest
'firm' is the definition.
(I've seen this before)
'answer man accepts' is the wordplay.
I cannot really understand how this works, but
'answer' could be 'a' (abbreviation eg in Q and A) and 'a' is found within the answer.
'man' could be 'ernest' (I've seen this before) and 'ernest' is present in the leftover letters.
This accounts for all the letters.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for earnest that I've seen before include "Serious; pledge" , "Oscar Wilde play 'The Importance of Being . . . . . . .'" , "promise" , "Solemn; pledge" , "Sincere, solemn" .)