Determined detectives brought into action (7)
I believe the answer is:
decided
'determined' is the definition.
('decide' can be a synonym of 'determine')
'detectives brought into action' is the wordplay.
'detectives' becomes 'CID' (abbreviation for Criminal Investigation Department).
'brought into' means one lot of letters goes inside another.
'action' becomes 'deed' (deed is a kind of action).
'cid' placed into 'deed' is 'DECIDED'.
(Other definitions for decided that I've seen before include "Resolved, settled" , "Categorical" , "Determined; resolute" , "Marked" , "Came to a resolution" .)