The most common and most likely answer to this crossword clue is the 6 letter word LEADED
I've seen this crossword clue in Independent
L
E
A
D
E
D
Buddy explains!
The straight part
We know that "with toxic additive" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case with toxic additive = leaded.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 LEADED mean?
LEADED verb- To cover, fill, or affect with lead
- To place leads between the lines of.
- Held in place by strips of lead.
- Containing or treated with the element lead.
- Containing tetraethyllead.