The most common and most likely answer to this crossword clue is the 9 letter word SUFFOCATE
I've seen this crossword clue in The Guardian
S
U
F
F
O
C
A
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E
Buddy explains!

The straight part
We know that "Kill" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case kill = suffocate.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 SUFFOCATE mean?
SUFFOCATE verb- To suffer, or cause someone to suffer, from severely reduced oxygen intake to the body.
- To die due to, or kill someone by means of, insufficient oxygen supply to the body.
- To overwhelm, or be overwhelmed (by a person or issue), as though with oxygen deprivation.
- To destroy; to extinguish.
- Suffocated; choked.