More drunk from barrelful drunk behind the bar (6)
I believe the answer is:
fuller
'more drunk from barrelful' is the definition.
I can't judge whether this defines the answer.
'drunk behind the bar' is the wordplay.
'drunk' becomes 'full' (full can mean drunk in Australian slang).
'behind' is a charade indicator (letters next to each other).
'the bar' becomes 'er' (I am not sure about this - if you are sure you should believe this answer much more).
'full'+'er'='FULLER'
Can you help me to learn more?
(Another definition for fuller that I've seen is " Less empty".)