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