Iscariot finally with motive for treachery (7)
I believe the answer is:
treason
'for treachery' is the definition.
(synonyms)
'iscariot finally with motive' is the wordplay.
'finally' suggests the final letters.
'with' is a charade indicator (letters next to each other).
'motive' becomes 'reason' (reason is a kind of motive).
The final letter of 'iscariot' is 't'.
't'+'reason'='TREASON'
(Other definitions for treason that I've seen before include "Crime of treachery against the state" , "Lese-majesty" , "Gunpowder Plot, say" , "Betrayal of one's nation" , "attempting to overthrow the king" .)