A saucer, perhaps from Ganymede? (9)

I believe the answer is:

cupbearer

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'a saucer' is the definition.
Although both the answer and definition are singular nouns, I can't understand how one could define the other.

I cannot understand how the remainder of the clue works.

Can you help me to learn more?

(Another definition for cupbearer that I've seen is " Hebe, perhaps".)

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