The most common and most likely answer to this crossword clue is the 4 letter word UPAS
I've seen this crossword clue in Financial Times
U
P
A
S
Buddy explains!

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 UPAS mean?
UPAS noun- (usually countable) A tree, Antiaris toxicaria, of the mulberry family, common in the forests of Java and the neighboring islands, with poisonous secretions.
- A virulent poison used in Java and the adjacent islands for poisoning arrows derived from the tree.
- A poison prepared from the climbing plant Strychnos tieute.