Find lie about non-believer (7)
I believe the answer is:
infidel
'non-believer' is the definition.
(I know that non-believer can be written as infidel)
'find lie about' is the wordplay.
'about' is an anagram indicator.
'find'+'lie'='findlie'
'findlie' with letters rearranged gives 'INFIDEL'.
(Other definitions for infidel that I've seen before include "One who doesn't believe" , "Sceptical" , "I don't believe" , "Person who has no faith, or the wrong one" , "Heathen" .)