All the best extras? (3-3)

I believe the answer is:

bye-bye

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

I don't understand the rest of the clue.

Can you help me to learn more?

(Other definitions for bye-bye that I've seen before include "Au revoir for juniors?" , "Ciao!" , "See you" , "Ta-ta" , "Adieu" .)

I've seen this clue in The Times.

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