Crossword Genius

Goodbye, twins! (2-2)

Ross

I believe the answer is:

ta-ta

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'goodbye' is the definition.
(I've seen this before)

I don't understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for ta-ta that I've seen before include "Bye!" , "Adieu" , "Bye-bye" , "A Farewell" , "Cheerio" .)

I've seen this clue in the Evening Standard.
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