Drunk spends billions in game of chance (5)
I believe the answer is:
lotto
'game of chance' is the definition.
(I've seen this in another clue)
'drunk spends billions' is the wordplay.
'drunk' becomes 'blotto' (slang term).
'spends' is a deletion indicator.
'billions' becomes 'b' (abbreviation).
'blotto' with 'b' taken out is 'LOTTO'.
'in' is the link.
(Other definitions for lotto that I've seen before include "Game in which numbers are drawn" , "Game like bingo" , "Lucky draw" , "Bingo - Venetian painter" , "Prize draw" .)