Our acorns cooked up bitter (9)
I believe the answer is:
rancorous
'bitter' is the definition.
(synonyms)
'our acorns cooked up' is the wordplay.
'cooked up' is an anagram indicator.
'our'+'acorns'='ouracorns'
'ouracorns' is an anagram of 'RANCOROUS'.
(Other definitions for rancorous that I've seen before include "spiteful?" , "Resentful, bitter" , "Showing bitter resentment" , "Acrimonious" .)