The most common and most likely answer to this crossword clue is the 7 letter word MIRAGES
I've seen this crossword clue in The Daily Telegraph
M
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R
A
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Buddy explains!

The straight part
We know that "Illusions" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case illusions = mirages.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 MIRAGES mean?
MIRAGES noun- An optical phenomenon in which light is refracted through a layer of hot air close to the ground, giving the appearance of there being refuge in the distance.
- An illusion.
- To cause to appear as or like a mirage.