VE Day overturned bitterness (5)
I believe the answer is:
venom
'bitterness' is the definition.
(thesaurus)
've day overturned' is the wordplay.
'day' becomes 'mon' (**).
'overturned' is a reversal indicator.
'mon' back-to-front is 'nom'.
've'+'nom'='VENOM'
(Other definitions for venom that I've seen before include "bitterness" , "acrimony" , "Snake's poison" , "Poisonous secretion" , "Virulent feeling" .)