Firm, with tenderness, managing to be convincing (8)

I believe the answer is:

cogently

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'managing to be convincing' is the definition.
I can't judge whether this defines the answer.

'firm with tenderness' is the wordplay.
'firm' becomes 'co' (a firm is a company).
'with tenderness' becomes 'gently' (I've seen this in another clue).
'co'+'gently'='COGENTLY'

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(Other definitions for cogently that I've seen before include "Persuasively, convincingly" , "with force" .)

I've seen this clue in the Sydney Morning Herald.

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