The most common and most likely answer to this crossword clue is the 15 letter word MISAPPREHENSION
I've seen this crossword clue in Independent
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More clues leading to the result MISAPPREHENSION
- Reportedly escape arrest? It’s a fallacy
- Cause of wrongful arrest?
- Some software about to get on top of woman in charitable centre? That’s a wrong understanding
- False impression made by undertaking absorbing little program about chicken
- Motorway signs finishing — fear poor understanding
- Fallacy about the foundation of Spanish Empire corrected
Buddy explains!

The straight part
We know that "Fallacy" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case fallacy = misapprehension.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 MISAPPREHENSION mean?
MISAPPREHENSION noun- A failure to understand something; an illusion, misconception or misunderstanding.