Fervent, perhaps militant, proponent (6)

I believe the answer is:

zealot

(Other definitions for zealot that I've seen before include "A fanatic or over-enthusiastic person" , "Fanatical believer" , "A fanatic or obsessed follower" , "Radical" , "Fanatic person, much too enthusiastic" .)

I've seen this clue in The Irish Times.

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