Belief in demigods, not God, strangely (5)
I believe the answer is:
deism
'belief' is the definition.
(deism is a kind of belief)
'demigods not god strangely' is the wordplay.
'not' indicates named letters should be taken away.
'strangely' indicates an anagram.
'demigods' with 'god' taken out is 'demis'.
'demis' is an anagram of 'DEISM'.
'in' acts as a link.
(Other definitions for deism that I've seen before include "Belief in a creator" , "Belief in the existence of God" , "Belief in a supreme being who does not intervene in the universe" , "Belief in a god from reason not revelation" .)