Sour or bitter (7)
I believe the answer is:
acerbic
I believe this is a double definition.
'sour' is the first definition.
(similar in meaning)
'bitter' is the second definition.
(synonyms)
(Other definitions for acerbic that I've seen before include "Sour in taste or harsh in tone" , "Cutting" , "Acrimonious" , "Sour; sharp" , "Sarcastic" .)