The most common and most likely answer to this crossword clue is the 4 plus 6 letter word AQUA TOFANA
A
Q
U
A
T
O
F
A
N
A
Buddy explains!
The straight part
We know that "oison" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case oison = aqua tofana.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.