Being bitter or sour can seem awful (9)
I believe the answer is:
rancorous
'being bitter' is the definition.
('rancorous' can be a synonym of 'bitter')
'or sour can seem awful' is the wordplay.
'seem awful' indicates anagramming the letters (I've seen this in other clues).
'or'+'sour'+'can'='orsourcan'
'orsourcan' with letters rearranged gives 'RANCOROUS'.
(Other definitions for rancorous that I've seen before include "Acrimonious" , "Showing bitter resentment" , "Resentful, bitter" , "spiteful?" .)