Fanatical adherent (6)
I believe the answer is:
zealot
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">fanatical adherent</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(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" .)