So long as end is gold plated (5)
I believe the answer is:
adieu
'so long' is the definition.
(synonyms)
'end is gold plated' is the wordplay.
'end' becomes 'die'.
'is 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).
'die' inserted inside 'au' is 'ADIEU'.
'as' is the link.
(Other definitions for adieu that I've seen before include "Farewel" , "12 Down" , "Gallic goodbye" , "Some say it when leaving" , "farewell message" .)