Cocktail drunk before dance (8)
I believe the answer is:
highball
'cocktail' is the definition.
(I've seen this before)
'drunk before dance' is the wordplay.
'drunk' becomes 'high' (I've seen this before).
'before' means one lot of letters go next to another.
'dance' becomes 'ball' (ball is a kind of dance).
'high'+'ball'='HIGHBALL'
(Other definitions for highball that I've seen before include "Whisky and soda with ice" , "Cocktail drunk" , "Spirit and soda in tall glass - lob (8);" , "Whiskey and soda with ice" , "Mixed alcoholic drink; go fast" .)