Crossword Genius

Farewell unduly protracted? (2,4)

Ross

I believe the answer is:

so long

I'm a little stuck... Click here to teach me more about this clue!

'farewell' is the definition.
(I know that farewell can be written as so long)

'unduly protracted?' is the wordplay.
I cannot really understand how this works, but
'protracted?' could be 'long' (similar in meaning) and 'long' is found within the answer.
The remaining letters 'so' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...

Can you help me to learn more?

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

I've seen this clue in The Guardian.
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