Belief is held by the opposition? (6)
I believe the answer is:
theism
'belief' is the definition.
(theism is a kind of belief)
'is held by the opposition?' is the wordplay.
'held by' means one lot of letters goes inside another.
'the opposition?' becomes 'them' (I can't explain this - if you can you should give a lot more credence to this answer).
'is' inserted within 'them' is 'THEISM'.
Can you help me to learn more?
(Other definitions for theism that I've seen before include "Belief in a creator" , "Belief in existence of a God revealed to man" , "of faith" , "The doctrine or belief in the existence of a God or gods" , "Belief in a deity" .)