Disbelief in God (7)
I believe the answer is:
atheism
'disbelief in god' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for atheism that I've seen before include "displaying opposition to religion" , "Owed" , "Non-belief in God" , "Lack of belief in the existence of God" , "Sceptic's position" .)