Company man is powerful (6)
I believe the answer is:
cogent
'powerful' is the definition.
(I've seen this before)
'company man' is the wordplay.
'company' becomes 'co' (abbreviation).
'man' becomes 'gent' (gent is a kind of man).
'co'+'gent'='COGENT'
'is' is the link.
(Other definitions for cogent that I've seen before include "Convincingly argued" , "Well-reasoned" , "Well argued, compelling" , "Forceful and convincing" , "'Powerfully persuasive, argument say (6)'" .)