Embittered (4)
I believe the answer is:
sour
'embittered' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for sour that I've seen before include "Acid tasting" , "Acetic" , "Sharp- or tart-tasting" , "Tasting sharp like stale milk" , "Rancid or tart" .)