Feel bitter at (6)
I believe the answer is:
resent
'feel bitter at' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for resent that I've seen before include "Feel bitter or indignant about" , "Don't like" , "Consider (something) to be an affront" , "Feel or show annoyance" , "Dispatched again; feel bitterness at" .)