The most common and most likely answer to this crossword clue is the 12 letter word SLAUGHTERING
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More clues leading to the result SLAUGHTERING
Buddy explains!

The straight part
We know that "killing" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case killing = slaughtering.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 SLAUGHTERING mean?
SLAUGHTERING verb- To butcher animals, generally for food
- To massacre people in large numbers
- To kill in a particularly brutal manner
- An act of slaughter; a killing.