Blue ones are fine (5)

I believe the answer is:

skies

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'blue ones' is the definition.
Although both the answer and definition are plural nouns, I cannot understand how one could define the other.

I cannot understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for skies that I've seen before include "Heavens" , "Hits high in the air" .)

I've seen this clue in the Evening Standard.

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