Tory rival (4)
I believe the answer is:
whig
'tory rival' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for whig that I've seen before include "Former 14 Down" , "Old-style politician" , "Seventeenth century politician" , "Member of a former reforming party" , "Former political party member" .)