Disciplinarian hit staff (6)
I believe the answer is:
ramrod
'disciplinarian' is the definition.
(ramrod can mean a harsh disciplinarian)
'hit staff' is the wordplay.
'hit' becomes 'ram' (ram can mean to hit something).
'staff' becomes 'rod' (both can mean a stick or cane).
'ram'+'rod'='RAMROD'
(Other definitions for ramrod that I've seen before include "Stern and inflexible person - part of a musket's equipment" , "Gun cleaner, proverbially stiff and straight" , "Someone stern and inflexible - it was used with a musket" , "Cannoneer's tool" , "Pole for pushing charge into a gun barrel" .)