May is not 1 stirring up bitterness (9)
I believe the answer is:
animosity
'bitterness' is the definition.
(thesaurus)
'may is not 1 stirring up' is the wordplay.
'1' becomes 'i' (roman numeral).
'stirring up' is an anagram indicator.
'may'+'is'+'not'+'i'='mayisnoti'
'mayisnoti' with letters rearranged gives 'ANIMOSITY'.
(Other definitions for animosity that I've seen before include "Ill-will" , "A spirit of hostility, enmity" , "Feeling of ill will and hostility" , "Maoist yin generates spirit of hostility" , "Hostile feeling" .)