Copper arresting current underworld boss (3)
I believe the answer is:
dis
'underworld boss' is the definition.
(Roman god of the underworld)
'copper arresting current' is the wordplay.
'copper' becomes 'ds' (detective sergeant).
'arresting' means one lot of letters goes inside another.
'current' becomes 'I' (physics symbol).
'ds' placed around 'i' is 'DIS'.
(Other definitions for dis that I've seen before include "Show contempt" , "Be rude" , "Hades" , "Name for Pluto or hell" , "soul venue" .)