Answer recognised when heard again (4)
I believe the answer is:
anew
'again' is the definition.
(I've seen this before)
'answer recognised when heard' is the wordplay.
'answer' becomes 'a' (abbreviation eg in Q and A).
'recognised' becomes 'knew' ('know' can be a synonym of 'recognize').
'when heard' indicates a 'sounds like' (homophone) clue.
'knew' sounds like 'new'.
'a'+'new'='ANEW'
(Other definitions for anew that I've seen before include "From the beginning again" , "Once more (poetic)" , "Wane in a different way, again" , "Afresh" , "Again but differently" .)