Republican's US rival (8)
I believe the answer is:
democrat
'republican's us rival' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for democrat that I've seen before include "Person who believes in government by and for the people" , "US political party supporter" , "US politician" , "Believer in government of, for and by the people" , "Believer in popular government" .)