Knight's title (3)
I believe the answer is:
'knight's title' is the definition.
(knights are addressed as 'sir')
This is all the clue.
(Other definitions for sir that I've seen before include "Title of Kt or Bart" , "Honourable title" , "Formal term of address" , "Poet and novelist" , "Formal and deferential address to a man" .)