Drunk losing head in gambling game (5)
I believe the answer is:
lotto
'gambling game' is the definition.
(I've seen this before)
'drunk losing head' is the wordplay.
'drunk' becomes 'blotto' (I've seen this before).
'losing head' means to remove the first letter.
'blotto' with its initial letter taken away is 'LOTTO'.
'in' acts as a link.
(Other definitions for lotto that I've seen before include "Game of chance involving balls" , "National game with large money prizes" , "Children's game; lottery" , "Children's game similar to bingo" , "gamble" .)