Firm of detectives in action (7)
I believe the answer is:
decided
'firm' is the definition.
(thesaurus)
'detectives in action' is the wordplay.
'detectives' becomes 'cid'.
'in' is an insertion indicator.
'action' becomes 'deed' (deed is a kind of action).
'cid' placed inside 'deed' is 'DECIDED'.
'of' acts as a link.
(Other definitions for decided that I've seen before include "Unquestionable" , "unmistakable" , "Resolved, settled" , "Categorical" , "Definite; agreed" .)