In fact he is truly a believer (6)
I believe the answer is:
'believer' is the definition.
(I know that theist is a type of believer)
'in fact he is truly a' is the wordplay.
'in' indicates a hidden word.
'THEIST' is hidden within 'fact he is truly a'.
(Other definitions for theist that I've seen before include "Tithes (anag.)" , "One who believe in a god" , "Person in good faith" , "Believer in the existence fo a god" , "One believes in the existence of a god or gods" .)