Wine is nastier when drunk (7)
I believe the answer is:
retsina
'wine' is the definition.
(type of Greek wine)
'nastier when drunk' is the wordplay.
'when drunk' is an anagram indicator (a drunken or disorderly form of the letters).
'nastier' anagrammed gives 'RETSINA'.
'is' is the link.
(Other definitions for retsina that I've seen before include "S European wine" , "Type of Greek wine" , "Oily-tasting Greek white wine" , "Greek resin-flavoured wine" , "Nastier (anag) - Greek wine" .)