Knight or teacher (3)
I believe the answer is:
sir
This clue is a double definition.
'knight' is the first definition.
(a knight has the title Sir)
'teacher' is the second definition.
(way of addressing a male teacher)
(Other definitions for sir that I've seen before include "as Winston Churchill was" , "Form of address for a knight" , "Noble title of undesirable kind" , "Baronet's title" , "Title for knight or teacher" .)