The most common and most likely answer to this crossword clue is the 4 letter word DILL
I've seen this crossword clue in The Guardian
D
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L
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The straight part
We know that "herb" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case herb = dill.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 DILL mean?
DILL noun- Anethum graveolens (the type species of the genus Anethum), a herb, the seeds of which are moderately warming, pungent, and aromatic, formerly used as a soothing medicine for children; also known as dillseed.
- A cucumber pickled with dill flavoring
- A fool.
- To cook or flavor with dill
- To still; to assuage; to calm; to soothe, as one in pain.