Polite form of address (3)
I believe the answer is:
sir
'polite form of address' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for sir that I've seen before include "Feted fellow" , "Form of address for a knight" , "Way to address gentleman" , "Gentleman's form of address" , "Polite form of address to a man" .)