Cryptic crossword clue
Killer of insects — one by mechanical process getting any number 8

The most common and most likely answer to this crossword clue is the 8 letter word ROTENONE

I've seen this crossword clue in The Guardian
R
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Buddy explains!

The straight part
We know that "Killer of insects" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case killer of insects = rotenone.
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 ROTENONE mean?

ROTENONE noun
  1. A toxic crystalline substance obtained from the roots of derris and related plants, widely used as an insecticide.

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