Act to keep police resolute (7)
I believe the answer is:
decided
'resolute' is the definition.
(resolute can mean decided or determined)
'act to keep police' is the wordplay.
'act' becomes 'deed' (deed is a kind of act).
'to keep' means one lot of letters goes inside another.
'police' becomes 'CID' (abbreviation for Criminal Investigation Department).
'deed' enclosing 'cid' is 'DECIDED'.
(Other definitions for decided that I've seen before include "Determined; resolute" , "Definite; agreed" , "Indisputable" , "Resolved, settled" , "Determined - settled" .)