Teacher to some extent rebuffed Christian (3)
I believe the answer is:
sir
'teacher' is the definition.
(way of addressing a male teacher)
'to some extent rebuffed christian' is the wordplay.
'rebuffed' shows that the letters should be reversed in order.
'to some extent' indicates a hidden word (I've seen 'extent of' mean this (I've seen this in other clues)).
'ris' is hidden within 'christian'.
'ris' backwards is 'SIR'.
(Other definitions for sir that I've seen before include "Title for knight or teacher" , "Knight's prefix" , "Polite or formal address to male" , "Poet and novelist" , "Title of knight or baronet" .)