Trendy sibling can sing finally, being firm (9)

I believe the answer is:

insisting

'being firm' is the definition.
('insist' can be a synonym of 'be firm')

'trendy sibling can sing finally' is the wordplay.
'trendy' becomes 'in' ('in' can mean fashionable).
'sibling' becomes 'sis' (short for sister).
'can' becomes 'tin' (both are metal containers).
'finally' says to take the final letters.
The last letter of 'sing' is 'g'.
'in'+'sis'+'tin'+'g'='INSISTING'

I've seen this clue in The Daily Express.

Want a hint initially instead of a full solution? Install my app