The most common and most likely answer to this crossword clue is the 10 letter word NIGHTSHADE
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The straight part
We know that "deadly one" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case deadly one = nightshade.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 NIGHTSHADE mean?
NIGHTSHADE noun- Any of the poisonous plants belonging to the genus Solanum, especially black nightshade or woody nightshade.
- Any plant of the wider Solanaceae family, including the nightshades as well as tomato, potato, eggplant, and deadly nightshade.
- Belladonna or deadly nightshade, Atropa belladonna.
- Any of several plants likened to nightshade, usually because of similar dark-colored berries.