Sound from police, perhaps, sealing off street in time (7)
I believe the answer is:
whistle
'sound' is the definition.
(whistling is a kind of sounding)
'street in time' is the wordplay.
'street' becomes 'st' (abbreviation).
'in' is an insertion indicator.
'time' becomes 'while' (a little while).
'st' put into 'while' is 'WHISTLE'.
'from police perhaps sealing off' is the link.
I am not very happy about this link. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for whistle that I've seen before include "something needing wetting?" , "Make cheerful sound" , "Shrill piping sound" , "Hits Lew to make shrill sound" , "Blow through teeth" .)