Teacher’s sign of agreement on return (3)
I believe the answer is:
don
'teacher's' is the definition.
(don is a kind of teacher)
'sign of agreement on return' is the wordplay.
'sign of agreement' becomes 'nod' (I've seen this before).
'on return' says the letters should be written in reverse.
'nod' reversed gives 'DON'.
(Other definitions for don that I've seen before include "Spanish male title" , "Yorkshire river" , "Put on; lecturer" , "and 13 Down: Mozart opera" , "Nod off like Mafia chief" .)