Lawman is firm on parking (3)
I believe the answer is:
cop
'lawman' is the definition.
(cop is a kind of lawman)
'firm on parking' is the wordplay.
'firm' becomes 'co' (abbreviation for company).
'on' means one lot of letters go next to another (in a down clue, letters appear on others).
'parking' becomes 'p' (abbreviation on road signs).
'co'+'p'='COP'
'is' is the link.
(Other definitions for cop that I've seen before include "catch" , "Short policeman" , "busy" , "Policeman (slang)" , "Police officer (slang)" .)