Adieu (2,4)

I think it could be either:

Which length description are you looking for?:

2,4

so long

'adieu' is the definition.
(synonyms)

This is all the clue.

(Other definitions for so long that I've seen before include "Informal goodbye; see you" , "Bye for now!" , "Ta-Ta!" , "Goodbye (colloq.)" , "Cheerio, bye-bye" .)

I've seen this clue in The Times and The Sunday Times.

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