Relates to two informers giving up Castor (6,4)

I believe the answer is:

binary star

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'castor' is the definition.
The answer and definition can be both natural objects as well as being singular nouns.
Perhaps you can see a link between them that I can't see?

I don't understand the rest of the clue.

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(Another definition for binary star that I've seen is " Dual sun system".)

I've seen this clue in the Irish Times.

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