Fervent partisan (6)
I believe the answer is:
zealot
'fervent partisan' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for zealot that I've seen before include "Fanatic or extreme enthusiast" , "Fanatic, usually religious" , "Fervent, even militant, proponent of something" , "Fanatic, fiery follower" , "One ardent for cause" .)