The most common and most likely answer to this crossword clue is the 8 letter word ATROPINE
I've seen this crossword clue in Financial Times
A
T
R
O
P
I
N
E
More clues leading to the result ATROPINE
- Poison developed from a protein
- Poison left to rise up in a wine unopened
- Think to take on spreading rat poison
- Drug in operation worked, though not the first
- Poison can make you a bit upset before long
- Left in denial after losing case over drug
- Poisonous compound produced from a synthetic protein
- A suspicious protein for poisoner
- Actors oddly suppose this is poisonous
- Die having first chewed old rat poison
Buddy explains!
The straight part
We know that "poison" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case poison = atropine.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 ATROPINE mean?
ATROPINE noun- An alkaloid extracted from the plant deadly nightshade (Atropa belladonna) and other sources, such as the seeds of the Thorn-apple. Though overdoses would be fatal it is used as a drug in medicine for its paralytic effects (e.g. in surgery to relax muscles, in dentistry to dry the mouth, in ophthalmology to dilate the pupils).