Gold rings fade for so long (5)
I believe the answer is:
adieu
'so long' is the definition.
('adieu' can be a synonym of 'so long')
'gold rings fade' is the wordplay.
'gold' becomes 'au' ('Au' can be a synonym of 'gold').
'rings' means one lot of letters goes inside another.
'fade' becomes 'die' (I can't justify this - if you can you should give a lot more credence to this answer).
'au' going around 'die' is 'ADIEU'.
'for' is the link.
Can you help me to learn more?
(Other definitions for adieu that I've seen before include "Fare thee well!" , "Gallic goodbye" , "Goodbye, mademoiselle" , "Parting greeting" , "Farewell to France" .)