This wine would struggle to be nastier (7)
I believe the answer is:
retsina
'this wine' is the definition.
(type of Greek wine)
'would struggle to be nastier' is the wordplay.
'would struggle to be' is an anagram indicator (letters struggle into a new form).
'nastier' anagrammed gives 'RETSINA'.
(Other definitions for retsina that I've seen before include "Oily-tasting Greek white wine" , "Nastier (anag) - Greek wine" , "Type of Greek wine" , "Greek wine - nastier sort" , "S European wine" .)