Firm with quiet officer (3)
I believe the answer is:
cop
'officer' is the definition.
(I know that cop is a type of officer)
'firm with quiet' is the wordplay.
'firm' becomes 'co' (a firm is a company).
'with' says to put letters next to each other.
'quiet' becomes 'p' (music symbol for 'piano', ie soft or quiet).
'co'+'p'='COP'
(Other definitions for cop that I've seen before include "Short policeman" , "An American policeman" , "Member of the police force and an arrest" , "Get" , "busy" .)