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 a hidden word.
'SIR' can be found hidden inside '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" .)