Judas's characteristic alternative (7)
I believe the answer is:
traitor
'judas's' is the definition.
(I've seen this before)
'characteristic alternative' is the wordplay.
'characteristic' becomes 'trait' (I've seen this before).
'alternative' becomes 'or'.
'trait'+'or'='TRAITOR'
(Other definitions for traitor that I've seen before include "Betrayer, deceiver" , "Double-crosser" , "defector?" , "Person betraying a cause" , "Betrayer, quisling" .)