Definite the police must go into action (7)
I believe the answer is:
decided
'definite' is the definition.
(I've seen this before)
'police must go into action' is the wordplay.
'police' becomes 'CID' (abbreviation for Criminal Investigation Department).
'must go into' means one lot of letters goes inside another.
'action' becomes 'deed' (deed is a kind of action).
'cid' going into 'deed' is 'DECIDED'.
'the' acts as a link.
(Other definitions for decided that I've seen before include "Definite; agreed" , "Determined - settled" , "Determined; resolute" , "Made up one's mind" , "Marked" .)