More unpleasant sort of retsina (7)
I believe the answer is:
nastier
'more unpleasant' is the definition.
('nasty' can be a synonym of 'unpleasant')
'sort of retsina' is the wordplay.
'sort of' indicates anagramming the letters.
'retsina' anagrammed gives 'NASTIER'.
(Other definitions for nastier that I've seen before include "Getting even worse" , "Less pleasant" , "More horrid" , "Less nice" , "more aggressive" .)