One not believing in attending robbery (7)
I believe the answer is:
atheist
'one not believing' is the definition.
(atheists don't believe in God)
'attending robbery' is the wordplay.
'attending' becomes 'at' (e.g. at school means attending school).
'robbery' becomes 'heist' (heist is a kind of robbery).
'at'+'heist'='ATHEIST'
'in' acts as a link.
(Other definitions for atheist that I've seen before include "Unfaithful type" , "Sceptic" , "One disbelieving in the existence of God or gods" , "One doesn't believe" , "Person who does not believe in existence of a god" .)