Firm answer Hemingway accepts? (7)
I believe the answer is:
earnest
'firm' is the definition.
(I've seen this before)
'answer hemingway accepts?' is the wordplay.
'answer' becomes 'a' (abbreviation eg in Q and A).
'hemingway' becomes 'Ernest Hemingway' (synonyms).
'accepts?' means one lot of letters goes inside another.
'a' going into 'ernest' is 'EARNEST'.
(Other definitions for earnest that I've seen before include "Sincere; determined" , "'Heartfelt, sincere (7)'" , "Serious-minded" , "Serious; down-payment" , "Seriously eager" .)