Telling man to support firm (6)
I believe the answer is:
cogent
'telling' is the definition.
(synonyms)
'man to support firm' is the wordplay.
'man' becomes 'gent' (gent is a kind of man).
'to support' is a charade indicator (letters next to each other) (in a down clue, some letters hold others up).
'firm' becomes 'co' (a firm is a company).
'gent' after 'co' is 'COGENT'.
(Other definitions for cogent that I've seen before include "Forceful - persuasive" , "Well-reasoned" , "Compelling or convincing" , "Clearly argued" , "'Powerfully persuasive, argument say (6)'" .)