Difficult illusion, variable (6)

I believe the answer is:

tricky

'difficult' is the definition.
(similar in meaning)

'illusion variable' is the wordplay.
'illusion' becomes 'trick' (synonyms).
'variable' becomes 'y' (algebra).
'trick'+'y'='TRICKY'

(Other definitions for tricky that I've seen before include "Thorny" , "Crafty, cunning" , "Problematic - deceitful" , "Involving snags" , "Crafty, cunnning" .)

I've seen this clue in The Daily Express.

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