Gold-plated tumbler must be off in Versailles (5)
I believe the answer is:
adieu
'off in versailles' is the definition.
The definition suggests a singular noun which matches the answer.
'gold-plated tumbler' is the wordplay.
'gold' becomes 'Au' (Au is the chemical symbol for gold).
'plated' means one lot of letters goes inside another (plated can mean covered or coated).
'tumbler' becomes 'die' (I can't justify this - if you can you should believe this answer much more).
'au' enclosing 'die' is 'ADIEU'.
'must be' is the link.
Can you help me to learn more?
(Other definitions for adieu that I've seen before include "Goodbye, mademoiselle" , "Farewell to France" , "Gallic goodbye" , "Farewell, ami!" , "Some say it when leaving" .)