The ultimate in daylight robbery for a believer (6)
I believe the answer is:
theist
'a believer' is the definition.
(theist is a kind of believer)
'the ultimate in daylight robbery' is the wordplay.
'the ultimate in' indicates one should take the final letters.
'robbery' becomes 'heist' (heist is a kind of robbery).
The final letter of 'daylight' is 't'.
't'+'heist'='THEIST'
'for' acts as a link.
(Other definitions for theist that I've seen before include "Believer in a god or gods" , "One believes in the existence of a god or gods" , "Tithes for believer in God" , "Believer in god/gods" , "One who believe in a god" .)