Masterly title (3)
I believe the answer is:
sir
'title' is the definition.
(respectful address for a man)
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for sir that I've seen before include "Polite form of address to a man" , "Gentleman's form of address" , "address to teacher" , "Polite or formal address to male" , "Formal term of address" .)