Castor is a heavenly one (4)

I believe the answer is:

twin

'one' is the definition.
(thesaurus)

I don't understand the rest of the clue.

(Other definitions for twin that I've seen before include "One of two conceived together" , "(Town) paired with another" , "One of two conceived simultaneously" , "Romulus or Remus" , "Castor or Pollux?" .)

I've seen this clue in the Evening Standard.

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