Disciplinarian with heavy, imposing presence at the outset (4)
I believe the answer is:
whip
'disciplinarian' is the definition.
(I have seen 'Parliamentary disciplinarian' mean 'whip' so perhaps 'disciplinarian' could also mean 'whip')
'with heavy imposing presence at the outset' is the wordplay.
'at the outset' says to take the initial letters (the outset is the beginning).
The initial letters of 'with heavy imposing presence' is 'WHIP'.
(Other definitions for whip that I've seen before include "Parliamentary disciplinarian" , "Strike with strap" , "Fluffy dessert" , "Instrument of punishment" , "MP enforcing discipline" .)