Gentleman (3)
I believe the answer is:
sir
'gentleman' is the definition.
(a man may be addressed as sir)
This is the entire clue.
(Other definitions for sir that I've seen before include "Polite or formal address to male" , "Baronet's title" , "Polite form of address to a man" , "Noble title of undesirable kind" , "Form of address for a knight" .)