Established act constraining police (7)
I believe the answer is:
decided
'established' is the definition.
(thesaurus)
'act constraining police' is the wordplay.
'act' becomes 'deed' (deed is a kind of act).
'constraining' indicates putting letters inside.
'police' becomes 'cid'.
'deed' placed around 'cid' is 'DECIDED'.
(Other definitions for decided that I've seen before include "Resolute; settled" , "Resolved, settled" , "Made mind up, being unequivocal" , "Categorical" , "Indisputable" .)