Title of Osiris (3)
I believe the answer is:
sir
'title' is the definition.
(respectful address for a man)
'of osiris?' is the wordplay.
'of' indicates the answer is hidden within the clue.
'SIR' is hidden in the letters of 'osiris'.
(Other definitions for sir that I've seen before include "address to teacher" , "Formal term of address" , "Gentleman's form of address" , "... ___ John Major" , "Feted fellow" .)