Obvious gain for Tory PM (7)
I believe the answer is:
baldwin
'tory pm' is the definition.
(Stanley Baldwin)
'obvious gain' is the wordplay.
'obvious' becomes 'bald'.
'gain' becomes 'win' (I've seen this before).
'bald'+'win'='BALDWIN'
'for' is the link.
(Other definitions for baldwin that I've seen before include "Former PM" , "British PM three times between WWI and WWII" , "British statesman" , "Prime minister" , "Medieval monarch - black American writer - British premier" .)