Churchill gains weight (7)
I believe the answer is:
winston
'churchill' is the definition.
(UK prime minister Winston Churchill)
'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 "-- Churchill, UK PM" , "Statesman" , "war leader" , "11 [LAWMAKER]" , "Man" .)