Comparatively unpleasant retsina that's drunk (7)
I believe the answer is:
nastier
'comparatively unpleasant' is the definition.
('nasty' can be a synonym of 'unpleasant')
'retsina that's drunk' is the wordplay.
'that's drunk' is an anagram indicator.
'retsina' with letters rearranged gives 'NASTIER'.
(Other definitions for nastier that I've seen before include "more awkward" , "More disgusting" , "Retsina (anag) -- not so nice" , "more aggressive" , "Less pleasant" .)