The most common and most likely answer to this crossword clue is the 13 letter word FALSIFICATION
I've seen this crossword clue in The Daily Telegraph
F
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Buddy explains!

The straight part
We know that "Deceit" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case deceit = falsification.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 FALSIFICATION mean?
FALSIFICATION noun- The act of falsifying, or making false; a counterfeiting; the giving to a thing an appearance of something which it is not
- Knowingly false statement or wilful misrepresentation
- Showing an item of charge in an account to be wrong