Resentful at beer (6)
I believe the answer is:
bitter
I believe this clue is a double definition.
'resentful' is the first definition.
('bitter' can be a synonym of 'resentful')
'beer' is the second definition.
(I've seen this before)
'at' is the link.
(Other definitions for bitter that I've seen before include "Unpalatable; a beer" , "Sarcastic (words); very cold" , "English beer, sounds sour" , "Beer; not sweet" , "Not sweet; very cold" .)