Came out on top, you say, in both halves (3)
I believe the answer is:
one
'both halves' is the definition.
(I've seen this before)
'came out on top you say' is the wordplay.
'came out on top' becomes 'won' (I've seen this in another clue).
'you say' indicates a 'sounds like' (homophone) clue.
'won' is a homophone of 'ONE'.
'in' acts as a link.
(Other definitions for one that I've seen before include "This is singular" , "Smallest whole number" , "Lowest cardinal number" , "Sole; number" , "A certain; single person" .)