The most common and most likely answer to this crossword clue is the 5 letter word TOXIN
I've seen this crossword clue in Financial Times
T
O
X
I
N
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 TOXIN mean?
TOXIN noun- A toxic or poisonous substance produced by the biological processes of biological organisms.