Lost top in drunken gamble (5)

I believe the answer is:

lotto

I'm a little stuck... Click here to teach me more about this clue!

'gamble' is the definition.
(I've seen this before)

'lost top in drunken' is the wordplay.
'top' indicates taking the first letters (in a down clue, the first letter is top).
'in drunken' becomes 'otto' (I can't justify this - if you can you should believe this answer much more).
The initial letter of 'lost' is 'l'.
'l'+'otto'='LOTTO'

Can you help me to learn more?

(Other definitions for lotto that I've seen before include "chancy business" , "Children's game similar to bingo" , "National gambling game" , "Prize draw" , "National game with large money prizes" .)

I've seen this clue in the Sydney Morning Herald.

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