Religious belief is held by our opponents (6)
I believe the answer is:
theism
'religious belief' is the definition.
(theism is a kind of religious belief)
'is held by our opponents' is the wordplay.
'held by' means one lot of letters goes inside another.
'our opponents' becomes 'them' (bridge term).
'is' put inside 'them' is 'THEISM'.
(Other definitions for theism that I've seen before include "Belief in a creator" , "of faith" , "The doctrine or belief in the existence of a God or gods" , "Belief in a deity" , "Belief in existence of a God revealed to man" .)