Alcoholic drink disappeared, tipped up — around two gallons (6)
I believe the answer is:
eggnog
'alcoholic drink' is the definition.
(type of alcoholic drink)
'disappeared tipped up around two gallons' is the wordplay.
'two gallons' means two substitutions for 'gallon'.
'disappeared' becomes 'gone' (going is a kind of disappearing).
'tipped up' is a reversal indicator (in a down clue, letters go up).
'around' means one lot of letters goes inside another.
'gallon' becomes 'g' (abbreviation for the unit of liquid).
'gallon' becomes 'g' (abbreviation for the unit of liquid).
'gone' written backwards gives 'enog'.
'g'+'g'='gg'
'enog' enclosing 'gg' is 'EGGNOG'.
(Other definitions for eggnog that I've seen before include "Milky winter punch" , "Punch comprising cream and other farm produce" , "Drink similar to advocaat" , "Drink (milk punch)" , "Hot alcoholic drink" .)