Cocktail skier (8)
I believe the answer is:
Here is my best explanation:
This might be a double definition.
'cocktail' is the first definition.
'skier' is the second definition.
Although both the answer and definition are singular nouns, I cannot understand how they can define each other.
(Other definitions for highball that I've seen before include "Mixed alcoholic drink; go fast", "Spirit and soda in tall glass - lob (8);", "Cocktail served in a tall glass", "Long drink of spirit and mixer", "speed?", "Cocktail drunk", "Whisky and soda with ice", "Beverage", "Drink" and "Iced alcoholic drink in tall glass".)