Belief is part of those who traditionally oppose us (6)
I believe the answer is:
theism
'belief is part of those who traditionally oppose us' is the definition.
'theism' can be an answer for 'belief' (theism is a kind of belief). I am not certain of the rest of the definition.
'belief is part of those' is the wordplay.
'belief' becomes 'is' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'is part of' indicates putting letters inside (part of the inside).
'those' becomes 'them'.
'is' going inside 'them' is 'THEISM'.
Can you help me to learn more?
(Other definitions for theism that I've seen before include "Belief in existence of a God revealed to man" , "Belief in a deity" , "of faith" , "Belief in a creator" , "The doctrine or belief in the existence of a God or gods" .)