Knight address (3)
I believe the answer is:
sir
'knight address' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for sir that I've seen before include "Formal term of address" , "Title of knight or baronet" , "as Winston Churchill was" , "Polite term of address for a man" , "Title of 1 across" .)