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' means one lot of letters go next to another.
'gathering up' indicates an anagram.
'stern' is an anagram of 'rnets'.
'co'+'rnets'='CORNETS'
'the' is the link.
(Other definitions for cornets that I've seen before include "Ice-cream cones" , "Brass instruments" .)