Blow the whistle on second jump (4)
I believe the answer is:
shop
'blow the whistle on' is the definition.
(both can mean to report or betray someone)
'second jump' is the wordplay.
'second' becomes 's' (common abbreviation).
'jump' becomes 'hop' (hop is a kind of jump).
's'+'hop'='SHOP'
(Other definitions for shop that I've seen before include "Be disloyal to" , "Retail premises" , "Store - grass up" , "Store - workplace" , "sales outlet" .)