Baronet's title (3)
I believe the answer is:
sir
'baronet's title' 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 "Baronet's title" , "Formal term of address" , "Respectful address for a man" , "Honorary male title" , "Polite term of address for a man" .)