Fanatical adherent (6)
I believe the answer is:
zealot
'fanatical adherent' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for zealot that I've seen before include "Person militant about a cause" , "Over-zealous person, fanatic" , "Fanatic or extreme enthusiast" , "Fanatical believer" , "Fervent advocate" .)