Heretic cheers over robbery (7)
I believe the answer is:
atheist
'heretic' is the definition.
(someone who doesn't believe in God)
'cheers over robbery' is the wordplay.
'cheers' becomes 'ta' (both can informally mean thank you).
'over' is a reversal indicator.
'robbery' becomes 'heist' (heist is a kind of robbery).
'ta' reversed gives 'at'.
'at'+'heist'='ATHEIST'
(Other definitions for atheist that I've seen before include "Disbeliever in deities" , "Person who does not believe in existence of a god" , "Disbeliever in gods" , "One believes there are no gods" , "Unfaithful type" .)