Drink demijohn in draught, knocking it all back (5)
I believe the answer is:
rioja
'drink' is the definition.
(Rioja is an example)
I don't understand how the remainder of the clue works.
(Other definitions for rioja that I've seen before include "Popular Spanish wine" , "Spanish red wine" , "wine region" , "Spanish wine variety" , "Spanish drink" .)