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 wordplay.
'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", "worshipping many gods" and "Worship of more than one god".)