The most common and most likely answer to this crossword clue is the 10 letter word HUMBUGGING
I've seen this crossword clue in The Guardian
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Buddy explains!
The straight part
We know that "being deceptive" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case being deceptive = humbugging.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 HUMBUGGING mean?
HUMBUGGING verb- To play a trick on someone, to cheat, to swindle, to deceive.
- (African American Vernacular) To fight; to act tough.
- To waste time talking.
- An act of one who humbugs (in all senses, for example, swindling, fighting, etc.).