Cheap wine (3,9)
'cheap wine' is the definition.
(plonk can informally mean cheap wine)
This is the entire clue.
(Other definitions for plonk that I've seen before include "Cheap wine (sl.)" , "Put down - cheap wine" , "Wine - put down with a thud" , "Cheap red or white" , "Ordinary wine" .)
(Other definitions for vin ordinaire that I've seen before include "this isn't the best drink" , "Plonk" , "Cheap table wine" .)