Answer put forward upset writer (5)
I believe the answer is:
aesop
'writer' is the definition.
(I've seen this before)
'answer put forward upset' is the wordplay.
'answer' becomes 'a' (abbreviation eg in Q and A).
'put forward' becomes 'pose' (I've seen this in another clue).
'upset' shows that the letters should be reversed in order.
'pose' in reverse letter order is 'esop'.
'a'+'esop'='AESOP'
(Other definitions for aesop that I've seen before include "story teller" , "Fable-teller" , "very old writer" , "Fabulous man" , "Greek author, famous for his fables" .)