How you regretted biting into the lemon? (8)
I believe the answer is:
bitterly
'how' is the definition.
Both the answer and definition are adverbs. Perhaps there's an association between them I don't understand?
'you regretted biting into the lemon?' is the wordplay.
I cannot really see how this works, but
'biting' could be 'bitter' (synonyms) and 'bitter' is found within the answer.
This may be the basis of clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for bitterly that I've seen before include "With resentment" , "Resentfully, rancorously" , "In a resentful manner" , "Sharply, pungently" , "Acrimoniously" .)