Knight's address (3)
I believe the answer is:
sir
I believe this clue is a double definition.
'knight's' is the first definition.
(a knight has the title Sir)
'address' is the second definition.
(I have seen 'formal address' mean 'sir' so perhaps 'address' could also mean 'sir')
(Other definitions for sir that I've seen before include "Respectful address for a man" , "Formal term of address" , "Feted fellow" , "Poet and novelist" , "Honourable title" .)