Nick to betray police station (3,4)
I believe the answer is:
cop shop
'police station' is the definition.
(I've seen this before)
'nick to betray' is the wordplay.
'nick' becomes 'cop' (both can mean to apprehend or arrest).
'to betray' becomes 'shop' (I've seen this before).
'cop'+'shop'='COP-SHOP'
(Other definitions for cop shop that I've seen before include "office investigating crime" , "Nick" , "'Police station (3,4)'" , "Police station (slang)" .)