A win for a second time (5)
I believe the answer is:
again
'second time' is the definition.
(again means for a second time)
'a win for a' is the wordplay.
'a win' becomes 'gain' (I've seen this before).
'for' is a charade indicator (letters next to each other) (I've seen this in other clues).
'gain' after 'a' is 'AGAIN'.
(Other definitions for again that I've seen before include "Moreover - a second time" , "Not for the first time" , "second term" , "One more time, encore" , "10 10 [AGAIN" .)