Farewell unduly protracted? (2,4)
I believe the answer is:
so long
'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" .)