Half a soft drink? (4)
I believe the answer is:
coca
'half a soft drink?' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for coca that I've seen before include "Leaves for snow" , "Drug-yielding shrub" , "Source of drug" , "S. American drug crop" , "Shrub yielding drug" .)