Knightly address (3)
I believe the answer is:
sir
'knightly address' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for sir that I've seen before include "Knight's title" , "... ___ John Major" , "Title given to 10 Across" , "Title of 1 across" , "Title of Kt or Bart" .)