More unpleasant than retsina? Unlikely! (7)
I believe the answer is:
nastier
'more unpleasant than' is the definition.
'retsina? unlikely' is the wordplay.
'unlikely' is an anagram indicator.
'retsina' anagrammed gives 'NASTIER'.
(Other definitions for nastier that I've seen before include "Less nice" , "More unpleasant or distasteful" , "more aggressive" , "More disgusting" , "more awkward" .)