Nine is the one to be on! (5)

I believe the answer is:

cloud

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'nine' is the definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.

I don't understand how the remainder of the clue works.

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(Other definitions for cloud that I've seen before include "Cover" , "Source of rain" , "Obscure" , "It hides the sun" , "Mass of vapour visible in the air" .)

I've seen this clue in the Evening Standard.

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