One to win over (5)
I believe the answer is:
again
'over' is the definition.
(I've seen this in another clue)
'one to win' is the wordplay.
'one' becomes 'a' (a thing is one thing).
'to win' becomes 'gain' (synonyms).
'a'+'gain'='AGAIN'
(Other definitions for again that I've seen before include "One more time, encore" , "Anew" , "Moreover - a second time" , "return" , "Next time" .)