Belief in one god (6)
I believe the answer is:
theism
'belief in one god' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for theism that I've seen before include "Belief in a deity" , "Belief in a creator" , "of faith" , "The doctrine or belief in the existence of a God or gods" , "Belief in existence of a God revealed to man" .)