The most common and most likely answer to this crossword clue is the 9 letter word UNREALITY
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More clues leading to the result UNREALITY
Buddy explains!

The straight part
We know that "illusion" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case illusion = unreality.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 UNREALITY mean?
UNREALITY noun- Lack of reality or real existence.
- The state of being unreal
- That which has no reality or real existence; something unreal or imaginary
- Unpractical character; visionariness.