The most common and most likely answer to this crossword clue is the 13 letter word PSEUDOSCIENCE
I've seen this crossword clue in Independent
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Buddy explains!

The straight part
We know that "Fake knowledge" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case fake knowledge = pseudoscience.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 PSEUDOSCIENCE mean?
PSEUDOSCIENCE noun- Any body of knowledge that purports to be scientific or to be supported by science but which fails to comply with the scientific method.