Mushroom drink (8)
I believe the answer is:
snowball
I believe this clue is a double definition.
'mushroom' is the first definition.
(both can mean to grow or accumulate)
'drink' is the second definition.
(type of cocktail)
(Other definitions for snowball that I've seen before include "Cold missile - advocaat and lemonade cocktail" , "Cocktail made from lemonade and advocaat" , "Cocktail; grow rapidly" , "Quickly get bigger" , "Cold sphere" .)