The most common and most likely answer to this crossword clue is the 6 letter word SEPTIC
I've seen this crossword clue in Independent
S
E
P
T
I
C
More clues leading to the result SEPTIC
- Affected by bugs, get cold after first day of month
- Nurses finally record twitching associated with bacteria
- Festering group in charge retaining power
- Infected by bacteria, Mick sheds skin after 30 days
- Bad quotes recalled, including one from Plato
- On the first of the month, Charlie got infected
- Poisonous priest among group in charge
- Rotting this way in record time? Contrariwise!
- Poisoned quotes protecting power on the rise
- Addled American celebrated by bards of Bow Bells
Buddy explains!

The straight part
We know that "Poisonous" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case poisonous = septic.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 SEPTIC mean?
SEPTIC noun- A substance that causes sepsis or putrefaction.
- A septic tank; a system for the disposal of sewage into a septic tank, a septic system.
- Of or pertaining to sepsis.
- Causing sepsis or putrefaction.
- Of or pertaining to sewage or the disposal of sewage.
- A mathematical object (function, curve, surface, etc.) of degree seven.
- Of the seventh degree or order.
- An American, a Yank.