Fervent proponent (6)
I believe the answer is:
zealot
'fervent proponent' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for zealot that I've seen before include "Fanatic, obsessed person" , "Fanatic, usually religious" , "One ardent for cause" , "Fanatic person, much too enthusiastic" , "Fanatical believer" .)