Nastier concoction of Greek wine (7)
I believe the answer is:
retsina
'greek wine' is the definition.
(I've seen this before)
'nastier concoction' is the wordplay.
'concoction' indicates anagramming the letters.
'nastier' with letters rearranged gives 'RETSINA'.
'of' is the link.
(Other definitions for retsina that I've seen before include "Oily-tasting Greek white wine" , "Nastier (anag) - white (or rose) wine" , "Greek resin-flavoured wine" , "Greek drink" , "Type of Greek wine" .)