Merely half-chill drink (5)
I believe the answer is:
juice
'drink' is the definition.
(I've seen this before)
'merely half-chill' is the wordplay.
'merely' becomes 'just' (synonyms).
'half' indicates taking half.
'chill' becomes 'ice' (icing is a kind of chilling).
'just' halved is 'ju'.
'ju'+'ice'='JUICE'
(Other definitions for juice that I've seen before include "It can be expressed" , "Fruit extract" , "Energy" , "Sap; petrol" , "fuel" .)