Police stopping game: important one (7)
I believe the answer is:
decider
'important one' is the definition.
The definition suggests a singular noun which matches the answer.
'police stopping game' is the wordplay.
'police' becomes 'CID' (abbreviation for Criminal Investigation Department).
'stopping' means one lot of letters goes inside another (stopping can mean blocking a gap).
'game' becomes 'deer' (type of game animal).
'cid' inserted within 'deer' is 'DECIDER'.
Can you help me to learn more?
(Other definitions for decider that I've seen before include "Replay perhaps" , "Umpire" , "Last game after a draw" , ""Golden goal", for example" , "vital match" .)