A large number, after festival drinking gallons, in a state once (4,7)
I believe the answer is:
east germany
'a state once' is the definition.
(former country)
'a large number after festival drinking gallons in' is the wordplay.
'a large number' becomes 'many' (many means lots of something).
'after' means one lot of letters go next to another.
'festival drinking' becomes 'easter' (I can't justify this - if you can you should give a lot more credence to this answer).
'gallons' becomes 'G' (abbreviation).
'in' indicates putting letters inside.
'easter' going around 'g' is 'eastger'.
'many' put after 'eastger' is 'EAST GERMANY'.
(Other definitions for east germany that I've seen before include "Former state" , "GDR, before 1990" .)