Metatarsal's so long (2-2)

I believe the answer is:

ta-ta

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'so long' is the definition.
(both are ways of saying goodbye)

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

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

I've seen this clue in The Independent.

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