Believer loses head in robbery (5)
I believe the answer is:
heist
'robbery' is the definition.
(heist is a kind of robbery)
'believer loses head' is the wordplay.
'believer' becomes 'theist' (theist is a kind of believer).
'loses head' means to remove the first letter.
'theist' with its initial letter removed is 'HEIST'.
'in' acts as a link.
(Other definitions for heist that I've seen before include "Armed robbery" , "criminal activity" , "Mugging" , "Audacious robbery" , "Bank robbery" .)