Teacher is taken aback, meeting resistance (3)
I believe the answer is:
sir
'teacher' is the definition.
(way of addressing a male teacher)
'is taken aback meeting resistance' is the wordplay.
'taken aback' says the letters should be written in reverse.
'meeting' is a charade indicator (letters next to each other).
'resistance' becomes 'r' (abbreviation in physics and electronics).
'is' written backwards gives 'si'.
'si'+'r'='SIR'
(Other definitions for sir that I've seen before include "Title given to 10 Across" , "Title of 1 across" , "Formal and deferential address to a man" , "Knight's prefix" , "Formal term of address" .)