'Firm' detectives caught in act (7)
I believe the answer is:
decided
'firm' is the definition.
(thesaurus)
'detectives caught in act' is the wordplay.
'detectives' becomes 'cid'.
'caught in' indicates putting letters inside.
'act' becomes 'deed' (deed is a kind of act).
'cid' inserted inside 'deed' is 'DECIDED'.
(Other definitions for decided that I've seen before include "Categorical" , "Resolute; settled" , "Came to a resolution" , "Made up one's mind" , "Resolved, settled" .)