The most common and most likely answer to this crossword clue is the 8 letter word WORMWOOD
I've seen this crossword clue in Independent
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The straight part
We know that "it’s a source of bitterness" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case it’s a source of bitterness = wormwood.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does WORMWOOD mean?
WORMWOOD noun- An intensely bitter herb (Artemisia absinthium and similar plants in genus Artemisia) used in medicine, in the production of absinthe and vermouth, and as a tonic.
- Something that causes bitterness or affliction; a cause of mortification or vexation.