Faithless fellow's missing at job (5)
I believe the answer is:
heist
'job' is the definition.
(thesaurus)
'faithless fellow's missing at' is the wordplay.
'faithless fellow' becomes 'atheist' (someone who doesn't believe in God).
'missing' suggests deleting specific letters.
'atheist' with 'at' removed is 'HEIST'.
(Other definitions for heist that I've seen before include "crime?" , "Audacious robbery" , "criminal activity" , "bank job in America?" , "Bank robbery" .)