Fervent proponent (6)
I believe the answer is:
zealot
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">fervent proponent</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 "Fanatic, obsessed person" , "Fanatic, usually religious" , "One ardent for cause" , "Fanatic person, much too enthusiastic" , "Fanatical believer" .)