I suppose you initially might (5)
I believe the answer is:
welly
'might' is the definition.
('welly' can mean a use of force or power)
'i suppose you initially' is the wordplay.
'i suppose' becomes 'well' (both are exclamations of irritation).
'initially' suggests taking the first letters.
The first letter of 'you' is 'y'.
'well'+'y'='WELLY'
(Other definitions for welly that I've seen before include "Oomph" , "showing great commitment" , "One going on foot" , "wet-weather gear" , "Rubber boot (abbr)" .)