Firm pressure on lawman (3)
I believe the answer is:
cop
'lawman' is the definition.
(cop is a kind of lawman)
'firm pressure on' is the wordplay.
'firm' becomes 'co' (a firm is a company).
'pressure' becomes 'p' (scientific abbreviation).
'on' is a charade indicator (letters next to each other).
'co'+'p'='COP'
(Other definitions for cop that I've seen before include "Policeman (informal)" , "Short policeman" , "Police officer (slang)" , "catch" , "Policeman (slang)" .)