Powerful company man (6)
I believe the answer is:
cogent
'powerful' is the definition.
(thesaurus)
'company man' is the wordplay.
'company' becomes 'co' (abbreviation).
'man' becomes 'gent' (gent is a kind of man**).
'co'+'gent'='COGENT'
(Other definitions for cogent that I've seen before include "Convincingly argued" , "Clear, logical and convincing" , "Compelling (argument)" , "Well-reasoned" , "Forceful and convincing" .)