One criminal originally arrested by European constabulary (6)
I believe the answer is:
police
'constabulary' is the definition.
(I know that constabulary can be written as police)
'one criminal originally arrested by european' is the wordplay.
'one' becomes 'i' (Roman numeral).
'originally' suggests taking the first letters.
'arrested by' means one lot of letters goes inside another (inserted letters are held or arrested).
'european' becomes 'pole' (someone from Poland).
The first letter of 'criminal' is 'c'.
'i'+'c'='ic'
'ic' going into 'pole' is 'POLICE'.
(Other definitions for police that I've seen before include "... custodians of law and order" , "Constabulary" , "Emergency service" , "Crime fighters" , "Gardai" .)