The most common and most likely answer to this crossword clue is the 12 letter word CHOLESTEROLS
I've seen this crossword clue in The Guardian
C
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Buddy explains!
The straight part
We know that "They might do you harm" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case they might do you harm = cholesterols.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 CHOLESTEROLS mean?
CHOLESTEROLS noun- A sterol lipid synthesized by the liver and transported in the bloodstream to the membranes of all animal cells; it plays a central role in many biochemical processes and, as a lipoprotein that coats the walls of blood vessels, is associated with cardiovascular disease.
- The level of cholesterol in the body.