Blow the whistle on store (4)
I believe the answer is:
shop
I believe this is a double definition.
'blow the whistle on' is the first definition.
(both can mean to report or betray someone)
'store' is the second definition.
(synonyms)
(Other definitions for shop that I've seen before include "Where goods are bought and sold" , "Inform on; store" , "Retail establishment" , "Retail outlet" , "Sell" .)