Win a gin cocktail (4)
I believe the answer is:
gain
'win' is the definition.
('gain' can be a synonym of 'win')
'a gin cocktail' is the wordplay.
'cocktail' indicates anagramming the letters.
'a'+'gin'='agin'
'agin' anagrammed gives 'GAIN'.
(Other definitions for gain that I've seen before include "Advance" , "Profit; acquire" , "Pick up" , "Benefit; put on (weight)" , "Obtain; benefit" .)