Not believing robbery left incomplete in high-level area (9)
I believe the answer is:
atheistic
'not believing' is the definition.
(I've seen this before)
'robbery left incomplete in high-level area' is the wordplay.
'robbery' becomes 'heist' (heist is a kind of robbery).
'left incomplete' means to remove the last letter.
'in' indicates putting letters inside.
'high-level area' becomes 'attic' (I can't justify this - if you can you should give a lot more credence to this answer).
'heist' with its last letter taken off is 'heis'.
'heis' put inside 'attic' is 'ATHEISTIC'.
Can you help me to learn more?
(Other definitions for atheistic that I've seen before include "Godless" , "Heathen" , "Not believing in god" , "rationalist philosophy?" , "denying God" .)