Stopped one who rejects accepted truth turning up (6)
I believe the answer is:
reined
'stopped' is the definition.
(to rein is to limit or stop)
'one who rejects accepted truth turning up' is the wordplay.
'one who rejects accepted truth' becomes 'denier'.
'turning up' says the letters should be written in reverse.
'denier' back-to-front is 'REINED'.
(Other definitions for reined that I've seen before include "Stopped or slowed a horse" , "Pulled in, checked progress" , "Restrained" .)