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