The most common and most likely answer to this crossword clue is the 6 letter word CURARE
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More clues leading to the result CURARE
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 = curare.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 CURARE mean?
CURARE noun- A plant, Strychnos toxifera, formerly used in arrow poisons in South America due to its D-tubocurarine content
- Other South American plants with similar toxins that were also used in arrow poisons, mostly in the family Menispermaceae
- A substance containing the alkaloid D-tubocurarine, used historically as a muscle relaxant during surgery