Find lie’s upset non-believer (7)
I believe the answer is:
infidel
'non-believer' is the definition.
(I've seen this before)
'find lie's upset' is the wordplay.
'upset' indicates an anagram.
'find'+'lie'='findlie'
'findlie' is an anagram of 'INFIDEL'.
(Other definitions for infidel that I've seen before include "Heathen" , "Sceptical" , "Person not operating in good faith" , "Person who has no faith, or the wrong one" , "Non-believer, heretic" .)