Policemen arresting Republican group (5)
I believe the answer is:
corps
'group' is the definition.
The answer is an example.
'policemen arresting republican' is the wordplay.
'policemen' becomes 'cops' (cop is a kind of policeman).
'arresting' means one lot of letters goes inside another.
'republican' becomes 'R' (abbreviation in US politics).
'cops' placed around 'r' is 'CORPS'.
(Other definitions for corps that I've seen before include "Body of men" , "Part of an army" , "Branch of an army" , "Group of soldiers" , "Large army group" .)