Action decided without detectives (4)
I believe the answer is:
deed
'action' is the definition.
(deed is a kind of action)
'decided without detectives' is the wordplay.
'without' suggests deleting specific letters.
'detectives' becomes 'CID' (abbreviation for Criminal Investigation Department).
'decided' with 'cid' removed is 'DEED'.
(Other definitions for deed that I've seen before include "Action - legal document" , "Feat" , "Act - exploit - legal document" , "Legal document for transfer of property" , "A conscious action" .)