Unorthodox believer (7)
I believe the answer is:
'unorthodox believer' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for heretic that I've seen before include "Unbeliever" , "Person in conflict with Roman Catholic dogma" , "Original thinker" , "One with unorthodox opinions" , "contrarian" .)