Knocks back sample of alcohol socialising in European capital (4)
I believe the answer is:
oslo
'european capital' is the definition.
(I've seen this before)
'knocks back sample of alcohol socialising' is the wordplay.
'knocks back' says the letters should be written in reverse (I've seen 'knock back' mean this).
'sample of' indicates the answer is hidden within the clue (I've seen this in other clues).
'olso' is hidden in the letters of 'alcohol socialising'.
'olso' reversed gives 'OSLO'.
'in' is the link.
(Other definitions for oslo that I've seen before include "where Ibsen lived under another name" , "Northern capital of Czechoslovakia?" , "Norwegian chief port" , "Norway's capital city" , "European capital city (found in Czechoslovakia!)" .)