Not happy with the beer (6)
I believe the answer is:
bitter
'beer' is the definition.
(I've seen this before)
'not happy with the' is the wordplay.
I cannot really see how this works, but
'the' could be 't' (the is pronounced as a 't' sound in some dialects) and 't' is present in the answer.
The remaining letters 'biter' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for bitter that I've seen before include "Having a sharp, acid taste" , "Sour like some English beer" , "Sour taste or resentful" , "Sarcastic (words); very cold" , "Sharp-tasting or resentful" .)