Drink served with marshmallows (5)
I believe the answer is:
cocoa
'drink served with marshmallows' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for cocoa that I've seen before include "Reddish brown" , "Hot chocolate drink" , "Popular chocolate drink" , "Chocolate powder" , "Powdered cacao seeds" .)