The most common and most likely answer to this crossword clue is the 8 letter word PHANTASM
I've seen this crossword clue in The Times
P
H
A
N
T
A
S
M
More clues leading to the result PHANTASM
- Page with complex maths hiding an illusion
- Has tan spoiled in afternoon shadow?
- In May, county transfixed by a spirit
- Vision of man’s path being troubled
- Awful ham, pants in horror film
- Worker taken in by PM has silly illusion
- Illusion deceptive PM has about worker
- Triumphant, as military displays spirit
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 = phantasm.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 PHANTASM mean?
PHANTASM noun- Something seen but having no physical reality; a phantom or apparition.