Drink half of just frozen water (5)
I believe the answer is:
juice
'drink' is the definition.
(I've seen this before)
'half of just frozen water' is the wordplay.
'half of' indicates taking half.
'frozen water' becomes 'ice' (I've seen this before).
'just' halved is 'ju'.
'ju'+'ice'='JUICE'
(Other definitions for juice that I've seen before include "Liquid from fruit and vegetable" , "Fruit extract" , "power" , "fuel" , "Fruit drink" .)