Muscles displayed by northern cops (4)
I believe the answer is:
nabs
'cops' is the definition.
('nab' can be a synonym of 'cop')
'muscles displayed by northern' is the wordplay.
'muscles' becomes 'abs' (I've seen this before).
'displayed by' says to put letters next to each other.
'northern' becomes 'n' (abbreviation e.g. N. Ireland).
'abs' put after 'n' is 'NABS'.
(Other definitions for nabs that I've seen before include "Does appropriate" , "Arrests" , "Takes or grabs suddenly" , "Collars" , "Catches in the act" .)