Belief that there is no God (7)
I believe the answer is:
atheism
'belief that there is no god' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for atheism that I've seen before include "Disbelief in gods" , "Owed" , "Non-belief in God" , "Disbelief in a god" , "A lack of faith" .)