The most common and most likely answer to this crossword clue is the 7 letter word ARSENIC
I've seen this crossword clue in The Guardian
A
R
S
E
N
I
C
More clues leading to the result ARSENIC
- Poison distributed in scare
- Scare in spilling poison
- As seen in chemical table, it’s a poison
- Poisonous element nicer, as improved
- Some coarse nickname that’s toxic?
- As in scare, distressed
- As magazine has article left out, one chapter added
- As Racine’s plays
- Toxic element in care homes finally rehabilitated
- As a rule Sun has nice characters upset
Buddy explains!
The straight part
We know that "Killer" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case killer = arsenic.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 ARSENIC mean?
ARSENIC noun- A nonmetallic chemical element (symbol As) with an atomic number of 33.
- A single atom of this element.
- Arsenic trioxide.
- Of or containing arsenic with a valence of 5.