Mister (3)
I believe the answer is:
sir
'mister' is the definition.
(both can be used to address a man)
This is all the clue.
(Other definitions for sir that I've seen before include "Gentleman's form of address" , "as Winston Churchill was" , "Title given to 10 Across" , "Baronet's title" , "Title of Kt or Bart" .)