He spoilt my irrational belief (10)
I believe the answer is:
polytheism
'belief' is the definition.
(polytheism is a kind of belief)
'he spoilt my irrational' is the wordplay.
'irrational' is an anagram indicator.
'he'+'spoilt'+'my'='hespoiltmy'
'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" , "Worship of more than one god" .)