Capacity to get on after bitterness? (6)
I believe the answer is:
gallon
'capacity' is the definition.
(I've seen this before)
'on after bitterness?' is the wordplay.
'after' says to put letters next to each other.
'bitterness?' becomes 'gall' (synonyms).
'on' put after 'gall' is 'GALLON'.
'to get' acts as a link.
(Other definitions for gallon that I've seen before include "Liquid measure" , "Imperial measure for liquids" , "Several drinks of beer" , "Imperial measure of capacity" , "8 pints (or ten in a hat!)" .)