Be about to drop off teacher going north (3)
I believe the answer is:
nod
'be about to drop off' is the definition.
'teacher going north' is the wordplay.
'teacher' becomes 'don' (don is a kind of teacher).
'going north' says the letters should be written backwards.
'don' in reverse letter order is 'NOD'.
(Other definitions for nod that I've seen before include "Indicate assent; make slip" , "Gesture of assent" , "Where Cain went to" , "Fellow that's set up number 500" , "Signify assent" .)