Firm fellow is clear and convincing (6)
I believe the answer is:
cogent
'clear and convincing' is the definition.
(I've seen this in another clue)
'firm fellow' is the wordplay.
'firm' becomes 'co' (a firm is a company).
'fellow' becomes 'gent' (synonyms).
'co'+'gent'='COGENT'
'is' is the link.
(Other definitions for cogent that I've seen before include "Well-reasoned" , "Forceful and convincing" , "Strong" , "Powerful and convincing, argument say" , "Convincing, persuasive" .)