Respectful title of Osiris (3)
I believe the answer is:
sir
'respectful title' is the definition.
(I've seen this in another clue)
'of osiris' is the wordplay.
'of' indicates the answer is hidden within the clue.
'SIR' can be found hidden inside 'osiris'.
(Other definitions for sir that I've seen before include "Poet and novelist" , "Noble title of undesirable kind" , "Title given to baronet" , "Knight's prefix" , "Male title of respect" .)