European seriously drunk in bar (12)
I believe the answer is:
Here is my best explanation:
'european' is the definition.
(Byelorussian is an example)
'seriously drunk in bar' is the subsidiary indication.
'drunk' is an anagram indicator.
'in' means one lot of letters goes inside another.
'bar' becomes 'ban' (I've seen this in other clues).
'seriously' anagrammed gives 'yelorussi'.
'yelorussi' placed within 'ban' is 'byelorussian'.