Firm with quiet lawman (3)
I believe the answer is:
cop
'lawman' is the definition.
(cop is a kind of lawman)
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for cop that I've seen before include "Policeman (colloq.)" , "Get" , "Policeman (informal)" , "Police officer (informal)" , "busy" .)