Cocktail served in a tall glass (8)
I believe the answer is:
highball
'cocktail served in a tall glass' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for highball that I've seen before include "speed?" , "Whisky and soda with ice" , "Mixed drink served in a tall glass" , "Beverage" , "Iced alcoholic drink in tall glass" .)