Company chap is effective (6)
I believe the answer is:
cogent
'effective' is the definition.
(synonyms)
'company chap' is the wordplay.
'company' becomes 'co' (abbreviation).
'chap' becomes 'gent' (both can mean a man).
'co'+'gent'='COGENT'
'is' is the link.
(Other definitions for cogent that I've seen before include "'Clear and logical, argument say (6)'" , "Convincingly argued" , "Strong" , "Clearly argued" , "Forceful - persuasive" .)