Farewell rendition by one singer no good (2,4)

I believe the answer is:

so long

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'farewell' is the definition.
(so long is a kind of farewell)

I can't explain the rest of the clue.

(Other definitions for so long that I've seen before include "Bye for now!" , "Au revoir" , "This extended" , "Ta-Ta!" , "Cheerio, bye-bye" .)

I've seen this clue in The Times.

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