Belief in crusade is mistaken (5)
I believe the answer is:
deism
'belief' is the definition.
(deism is a kind of belief)
'in crusade is mistaken' is the wordplay.
'in' indicates the answer is hidden within the clue.
'DEISM' is hidden in the letters of 'crusade is mistaken'.
(Other definitions for deism that I've seen before include "Belief in a god from reason not revelation" , "Belief in a creator" , "Belief in the existence of God" , "Belief in a supreme being who does not intervene in the universe" .)