The most common and most likely answer to this crossword clue is the 10 letter word TOADSTOOLS
I've seen this crossword clue in Financial Times
T
O
A
D
S
T
O
O
L
S
More clues leading to the result TOADSTOOLS
Buddy explains!

The straight part
We know that "They’re poisonous" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case they’re poisonous = toadstools.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 TOADSTOOLS mean?
TOADSTOOLS noun- Any inedible or poisonous mushroom, especially an agaric.