Well-planned robbery is on time, I believe (6)
I believe the answer is:
theist
'i believe' is the definition.
(I've seen this before)
'well-planned robbery is on time' is the wordplay.
I cannot really understand how this works, but
'robbery' could be 'heist' (heist is a kind of robbery) and 'heist' is found in the answer.
'time' could be 't' (abbreviation) and 't' is present in the answer.
No letters remain.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for theist that I've seen before include "Religious type" , "Believer in god/gods" , "One who believe in a god" , "Tithes for believer in God" , "Person in good faith" .)