The most common and most likely answer to this crossword clue is the 5 letter word SARIN
I've seen this crossword clue in The Guardian
S
A
R
I
N
Buddy explains!

The straight part
We know that "there’s poison there /there’s poison there" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case there’s poison there /there’s poison there = sarin.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 SARIN mean?
SARIN noun- The neurotoxin O-isopropyl methylphosphonofluoridate, used as a chemical weapon.