Firm and stern, gathering up the instruments (7)
I believe the answer is:
cornets
'instruments' is the definition.
(cornet is a kind of instrument)
'firm and stern gathering up' is the wordplay.
'firm' becomes 'co' (a firm is a company).
'and' says to put letters next to each other.
'gathering up' indicates anagramming the letters.
'stern' anagrammed gives 'rnets'.
'co'+'rnets'='CORNETS'
'the' acts as a link.
(Other definitions for cornets that I've seen before include "Ice-cream cones" , "Brass instruments" .)