Retsina shaken is even worse (7)
I believe the answer is:
nastier
'even worse' is the definition.
(I've seen this before)
'retsina shaken' is the wordplay.
'shaken' indicates an anagram.
'retsina' anagrammed gives 'NASTIER'.
'is' is the link.
(Other definitions for nastier that I've seen before include "Less nice" , "More disgusting" , "Retsina (anag) -- not so nice" , "becomes more unpleasant" , "Getting even worse" .)