Arrest using firm pressure (3)
I believe the answer is:
cop
'arrest' is the definition.
(I know that arrest can be written as cop)
'firm pressure' is the wordplay.
'firm' becomes 'co' (a firm is a company).
'pressure' becomes 'p' (abbreviation).
'co'+'p'='COP'
'using' acts as a link.
(Other definitions for cop that I've seen before include "Police officer (informal)" , "An American policeman" , "Policeman (slang)" , "Get" , "Member of the police force and an arrest" .)