Betrayer of the Moor (4)
I believe the answer is:
iago
'betrayer of the moor' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for iago that I've seen before include "Evil character in Othello" , "Organ stop" , "villain in play" , "villainous ensign" , "Othello's evil counsellor" .)