He gains weight (7)
I believe the answer is:
winston
'he' is the definition.
'gains weight' is the wordplay.
'gains' becomes 'wins' ('win' can be a synonym of 'gain').
'weight' becomes 'ton' (unit of weight).
'wins'+'ton'='WINSTON'
(Other definitions for winston that I've seen before include "11 [LAWMAKER]" , "Man" , "Prime minister" , "war leader" , "-- Churchill, UK PM" .)