Ta-ra! (informal) (2,4)

I believe the answer is:

so long

(Other definitions for so long that I've seen before include ". . . . . ., Farewell', song from The Sound of Music" , "Goodbye (colloq.)" , "Cheerio, bye-bye" , "Informal goodbye; see you" , "Informal goodbye - protracted?" .)

I've seen this clue in The Guardian.

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