Cocktail cooler (3)
I believe the answer is:
ice
'cocktail cooler' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for ice that I've seen before include "Glaze" , "Diamonds (colloq.)" , "Form of water" , "Finish decorating the Christmas cake" , "Diamonds, colloquially" .)