Very protracted farewell? (2,4)

I believe the answer is:

so long

'farewell' is the definition.
(I know that so long is a type of farewell)

'very protracted' is the wordplay.
'very' becomes 'so' (both can mean 'extremely').
'protracted' becomes 'long-' (similar in meaning).
'so'+'long'='SO LONG!'

(Other definitions for so long that I've seen before include "Ta-Ta!" , "Au revoir" , "Slog on, conveying informal farewell" , "Informal goodbye; see you" , "This extended" .)

I've seen this clue in The Guardian, The Sun and The Times.

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