Teacher sounds less than assertive (3)
I believe the answer is:
sir
'teacher' is the definition.
(way of addressing a male teacher)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for sir that I've seen before include "Feted fellow" , "Male form of address" , "Title given to baronet" , "Title of knight or baronet" , "Term of address for a gentleman" .)