In class I respectfully address the teacher (3)
I believe the answer is:
'teacher' is the definition.
(way of addressing a male teacher)
'in class i respectfully address' is the wordplay.
'in' indicates the answer is hidden within the clue.
'SIR' can be found hidden inside 'class i respectfully address'.
'the' acts as a link.
(Other definitions for sir that I've seen before include "Title of 1 across" , "Polite term of address for a man" , "... ___ John Major" , "Formal term of address" , "Honourable title" .)