"My dear fellow" (3)
I believe the answer is:
sir
'my dear fellow' 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 "Knight's title" , "Title given to 10 Across" , "Respectful address for a man" , "Noble title of undesirable kind" , "address to teacher" .)