Convincingly firm without using force (8)
I believe the answer is:
cogently
'convincingly' is the definition.
(I've seen this before)
'firm without using force' is the wordplay.
'firm' becomes 'co' (a firm is a company).
'without using force' becomes 'gently' (gently does it).
'co'+'gently'='COGENTLY'
(Other definitions for cogently that I've seen before include "with force" , "Persuasively, convincingly" .)