Judas, corrupt one with cunning turned back (7)
I believe the answer is:
traitor
'judas' is the definition.
(I've seen this before)
'corrupt one with cunning turned back' is the wordplay.
'corrupt' becomes 'rot'.
'one with' becomes 'i' (I've seen this in other clues**).
'cunning' becomes 'art' (I've seen this before).
'turned back' says the letters should be written in reverse.
'rot'+'i'+'art'='rotiart'
'rotiart' in reverse letter order is 'TRAITOR'.
(Other definitions for traitor that I've seen before include "Betrayer of a country" , "Betrayer of a cause" , "Betrayer, deceiver" , "Turncoat" , "Double-crosser" .)