Go with gold plating for so long (5)
I believe the answer is:
adieu
'so long' is the definition.
(both can mean goodbye)
'go with gold plating' is the wordplay.
I cannot quite understand how this works, but
'go' could be 'die' (synonyms) and 'die' is found in the answer.
'gold' could be 'au' (Au is the chemical symbol for gold) and 'au' is present in the remaining letters.
No letters remain.
This explanation may well be incorrect...
'for' acts as a link.
Can you help me to learn more?
(Other definitions for adieu that I've seen before include "Gallic goodbye" , "Parting word" , "cheers" , "See you later" , "Parting greeting" .)