The most common and most likely answer to this crossword clue is the 8 letter word QUACKING
I've seen this crossword clue in The Daily Telegraph
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A
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Buddy explains!
The straight part
We know that "Don’t trust charlatan doing this" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case don’t trust charlatan doing this = quacking.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 QUACKING mean?
QUACKING verb- To make a noise like a duck.
- To practice or commit quackery (fraudulent medicine).
- To make vain and loud pretensions.
- The sound made by a group of ducks; quacks.
- Making quacking sounds, or prone to doing so.