Bitterness, rancour (4)
I believe the answer is:
gall
I believe this is a double definition.
'bitterness' is the first definition.
(I know that bitterness can be written as gall)
'rancour' is the second definition.
(gall can mean rancour or resentment)
(Other definitions for gall that I've seen before include "Irritate - presumption" , "Impudence, cheek" , "Ill-will - bile" , "Audacity" , "Impudence; bitterness" .)