The most common and most likely answer to this crossword clue is the 5 plus 8 letter word FOOLS PARADISE
I've seen this crossword clue in The Daily Telegraph
F
O
O
L
S
P
A
R
A
D
I
S
E
More clues leading to the result FOOLS PARADISE
- Cool about holding box by broken side? Content here is not secure
- State that may be so ideal for sap?
- Unrealistic place is a real spoof, concocted with a hint of devilment
- Enemy captured sailors’ pad, shattering false hopes and expectations
- Plead for Oasis to reform? It’s a fond illusion
- Illusion loads of praise must shatter
- Proof assailed in a make believe world
- Plead for Oasis to reform? It's a fond illusion
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 = fools paradise.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.