Vacuous Etonian tucking into relaxing bitter (9)
I believe the answer is:
resentful
'bitter' is the definition.
('resentful' can be a synonym of 'bitter')
'vacuous etonian tucking into relaxing' is the wordplay.
'vacuous' says to hollow out the word (remove centre letters).
'tucking into' is an insertion indicator.
'relaxing' becomes 'restful' ('restful' can be a synonym of 'relaxing'**).
'etonian' with its centre taken out is 'en'.
'en' inserted into 'restful' is 'RESENTFUL'.
(Other definitions for resentful that I've seen before include "Grudging" , "Taking ill" , "feeling sore" , "Bearing a grudge" , "Embittered - nurse left (anag)" .)