Drink, which is sad (4)
I believe the answer is:
down
I believe this clue is a double definition.
'drink' is the first definition.
(to down something can mean to drink it quickly)
'sad' is the second definition.
(down can mean sad or unhappy)
'which is' is the link.
(Other definitions for down that I've seen before include "Soft feathers (like this clue)" , "See 18" , "Depressed - feathers" , "when in adversity" , "In a low mood; hill" .)