Rarest moon moving for one? (10)

I believe the answer is:

astronomer

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'one?' is the definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.

'rarest moon moving' is the wordplay.
'moving' indicates an anagram.
'rarest'+'moon'='rarestmoon'
'rarestmoon' with letters rearranged gives 'ASTRONOMER'.

'for' is the link.

Can you help me to learn more?

(Other definitions for astronomer that I've seen before include "Observer of space" , "Moon-starer (anag)" , "Moon starer (anag.)" , "Dorado may be contemplated by me" , "Space studier" .)

I've seen this clue in The Telegraph.

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