Holy believer initially thwarts robbery (6)
I believe the answer is:
theist
'holy believer' is the definition.
(someone who believes in a god)
'initially thwarts robbery' is the wordplay.
'initially' suggests taking the first letters.
'robbery' becomes 'heist' (heist is a kind of robbery).
The initial letter of 'thwarts' is 't'.
't'+'heist'='THEIST'
(Other definitions for theist that I've seen before include "Person in good faith" , "Believer in a creator god" , "One believes in the existence of a god or gods" , "Believer in god/gods" , "Believer in a god or gods" .)