"Othello" role
I believe the answer is:
'othello role' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for cassio that I've seen before include "Michael in play" , "One accused of being Desdemona's lover (Othello)" , "Sheath" , "Character in Othello" , "Friend of Othello (Shak.)" .)