He spoilt my irrational belief (10)
I believe the answer is:
Here is my best explanation:
'belief' is the definition.
(polytheism is a kind of belief)
'he spoilt my irrational' is the subsidiary indication.
'irrational' is an anagram indicator.
'hespoiltmy' is an anagram of 'polytheism'.
(Other definitions for polytheism that I've seen before include "Belief in more than one god" and "Worship of more than one god".)