The ‘up’ side? (5)

I believe the answer is:

sunny

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'the up side?' is the definition.
Both the answer and definition are adjectives. Perhaps you can see an association between them that I can't see?

This is all the clue.

Can you help me to learn more?

(Other definitions for sunny that I've seen before include "Genial, cheerful" , "(Of a person's disposition) always positive" , "Cheerful; fine (weather)" , "Unclouded - cheerful" , "Bright and warm" .)

I've seen this clue in the Evening Standard.

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