Legal document decided police must leave (4)
I believe the answer is:
deed
'legal document' is the definition.
(deed is a kind of legal document)
'decided police must leave' is the wordplay.
'police' becomes 'cid'.
'must leave' suggests deleting specific letters.
'decided' with 'cid' taken out is 'DEED'.
(Other definitions for deed that I've seen before include "Something done" , "Intentional action" , "documentary evidence" , "Legal document for transfer of property" , "Action - legal document" .)