Retsina can be even worse! (7)
I believe the answer is:
nastier
'even worse' is the definition.
(I've seen this before)
'retsina can be' is the wordplay.
'can be' is an anagram indicator.
'retsina' with letters rearranged gives 'NASTIER'.
(Other definitions for nastier that I've seen before include "becomes more unpleasant" , "Retsina (anag) -- not so nice" , "more aggressive" , "More disgusting" , "More horrid" .)