Title held by Osiris (3)
I believe the answer is:
'title' is the definition.
(I've seen this before)
'held by osiris' is the wordplay.
'held by' says the answer is hidden in the clue.
'SIR' is hidden in the letters of 'osiris'.
(Other definitions for sir that I've seen before include "Polite or formal address to male" , "Knight's title" , "Noble title of undesirable kind" , "Way to address gentleman" , "Respectful address for a man" .)