Quiet men, trembling, holding quiet instruments (9)
I believe the answer is:
equipment
'instruments' is the definition.
(both can mean tools or devices)
'quiet men trembling holding quiet' is the wordplay.
'trembling' indicates anagramming the letters.
'holding' is an insertion indicator.
'quiet' becomes 'p' (music symbol for 'piano', ie soft or quiet).
'quiet'+'men'='quietmen'
'quietmen' is an anagram of 'equiment'.
'equiment' enclosing 'p' is 'EQUIPMENT'.
(Other definitions for equipment that I've seen before include "Kit, apparatus" , "Stuff" , "Apparatus, tools" , "4 [TACKLE]" , "Set of instruments that perform a service" .)