Powerful, firm guy (6)
I believe the answer is:
cogent
'powerful' is the definition.
(I've seen this before)
'firm guy' is the wordplay.
'firm' becomes 'co' (a firm is a company).
'guy' becomes 'gent' (I've seen this in another clue).
'co'+'gent'='COGENT'
(Other definitions for cogent that I've seen before include "Forceful and to the point" , "Clearly argued" , "Forceful and convincing" , "Forcefully convincing" , "Strong" .)