Lawman takes firm pressure (3)
I believe the answer is:
cop
'lawman' is the definition.
(cop is a kind of lawman)
'firm pressure' is the wordplay.
'firm' becomes 'co' (a firm is a company).
'pressure' becomes 'p' (scientific abbreviation).
'co'+'p'='COP'
'takes' is the link.
(Other definitions for cop that I've seen before include "Get" , "catch" , "Police officer (slang)" , "Member of the police force and an arrest" , "Policeman (slang)" .)