Gold in decline? Just the opposite for so long (5)
I believe the answer is:
adieu
'for so long' is the definition.
(both can mean goodbye)
'gold in decline? just the opposite' is the wordplay.
I cannot really understand how this works, but
'decline?' could be 'die' (I've seen this in other clues) and 'die' is present in the answer.
'gold' could be 'au' (Au is the chemical symbol for gold) and 'au' is found in the remaining letters.
No letters remain.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for adieu that I've seen before include "farewell message" , "cheers" , "Gallic goodbye" , "Goodbye, mon ami!" , "Cheerio" .)