Determined detectives in action (7)
I believe the answer is:
decided
'determined' is the definition.
('decide' can be a synonym of 'determine')
'detectives in action' is the wordplay.
'detectives' becomes 'CID' (abbreviation for Criminal Investigation Department).
'in' indicates putting letters inside.
'action' becomes 'deed' (deed is a kind of action).
'cid' inserted into 'deed' is 'DECIDED'.
(Other definitions for decided that I've seen before include "Marked" , "Resolute; settled" , "Determined - settled" , "Categorical" , "Indisputable" .)