A win? Not once (5)
I believe the answer is:
again
'once' is the definition.
Both the answer and definition are adverbs. Perhaps there's a link between them I don't understand?
'a win?' is the wordplay.
'win?' becomes 'gain' (synonyms).
'a'+'gain'='AGAIN'
'not' acts as a link.
Can you help me to learn more?
(Other definitions for again that I've seen before include "repeat order" , "Here we go ___" , "On the other hand" , "Anew" , "Next time" .)