In colour, this might be unrealistic (5)

I believe the answer is:

dream

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But I can't explain it at all! Could you help me understand?

(Other definitions for dream that I've seen before include "Cherished hope or ideal" , "It's experienced during sleep" , "Fantasy; imagine" , "Fantastic but vain hope" , "Vision, fantasy" .)

I've seen this clue in The Hindu.

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