His mate turned in disbelief (7)
I believe the answer is:
atheism
'disbelief' is the definition.
(thesaurus)
'his mate turned' is the wordplay.
'turned' indicates anagramming the letters.
'his'+'mate'='hismate'
'hismate' with letters rearranged gives 'ATHEISM'.
'in' acts as a link.
(Other definitions for atheism that I've seen before include "Sceptic's position" , "Disbelief in gods" , "A lack of faith" , "Lack of belief in the existence of God" , "displaying opposition to religion" .)