Soldiers and police arresting rook (5)
I believe the answer is:
corps
'soldiers' is the definition.
(a group of soldiers)
'police arresting rook' is the wordplay.
'police' becomes 'cops' (cop can informally mean a police officer).
'arresting' means one lot of letters goes inside another.
'rook' becomes 'R' (chess abbreviation).
'cops' placed around 'r' is 'CORPS'.
'and' acts as a link.
(Other definitions for corps that I've seen before include "Army division" , "gang" , "Large army group" , "Part of an army" , "Military body" .)