Teacher is following — O God! (6)
I believe the answer is:
osiris
'god' is the definition.
(Egyptian god)
'teacher is following o' is the wordplay.
'teacher' becomes 'sir' (way of addressing a male teacher).
'following' is a charade indicator (letters next to each other).
'sir'+'is'='siris'
'siris' after 'o' is 'OSIRIS'.
(Other definitions for osiris that I've seen before include "Husband of Isis, father of Horus - sis or I (anag)" , "Egyptian god - sis or I (anag)" , "Egyptian god of the dead" , "Egyptian god, father of Horus" , "underworld boss" .)