Disciplinarian (8)
I believe the answer is:
martinet
'disciplinarian' is the definition.
(I know that disciplinarian can be written as martinet)
This is all the clue.
(Other definitions for martinet that I've seen before include "Person who demands strict obedience" , "Tyrant, one who demands strict obedience" , "Gradgrind was one" , "Strict disciplinarian - met train (anag)" , "Disciplined type" .)