Term of respect (6)
'term of respect' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for sir that I've seen before include "Term of address for a gentleman" , "Respectful address for a man" , "Knight's title" , "Honorary male title" , "Knight's prefix" .)
(Other definitions for mister that I've seen before include "Bloke" , "Mr" , "Form of address to a man, used without his name" , "that's what you'd call him" , "Male form of address" .)