How one regretted sucking the lemon? (8)

I believe the answer is:

bitterly

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'how' is the definition.
Both the definition and answer are adverbs. Perhaps there's a link between them I don't understand?

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(Other definitions for bitterly that I've seen before include "Resentfully, rancorously" , "With resentment" , "Acrimoniously" , "Sharply, pungently" , "In a resentful manner" .)

I've seen this clue in the Evening Standard.

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