Sir dealt with cheats, fearsome disciplinarian? (9)
I believe the answer is:
chastiser
'fearsome disciplinarian?' is the definition.
'sir dealt with cheats' is the wordplay.
'dealt with' indicates anagramming the letters.
'sir' anagrammed with 'cheats' gives 'CHASTISER'.
(Other definitions for chastiser that I've seen before include "smacker" , "One who severely reprimands" , "One strapping" .)