A sunny expression (5)

I believe the answer is:

solar

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'a sunny' is the definition.
(I've seen this in another clue)

I can't explain the rest of the clue.

(Other definitions for solar that I've seen before include "Concerning the sun" , "room" , "Coming from the sun" , "Using the energy of the sun" , "Pertaining to the sun" .)

I've seen this clue in The Globe and Mail.

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