At such length, farewell! (2,4)
I believe the answer is:
so long
'farewell' is the definition.
(I know that so long is a type of farewell)
'at such length' is the wordplay.
I cannot quite understand how this works, but
'at' could be 'on' (location) and 'on' is present in the answer.
'such' could be 'so' and 'so' is located in the answer.
'length' could be 'l' (abbreviation in geometry) and 'l' is found within the answer.
A single letter 'g' remains 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" , "That length - goodbye!" , "Informal goodbye; see you" , "Au revoir" , "Adieu" .)