Resolute detectives in action (7)
I believe the answer is:
decided
'resolute' is the definition.
(resolute can mean decided or determined)
'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' going within 'deed' is 'DECIDED'.
(Other definitions for decided that I've seen before include "Came to a resolution" , "Made mind up, being unequivocal" , "Categorical" , "unmistakable" , "Determined; resolute" .)