Gold-plated cube is so long (5)
I believe the answer is:
adieu
'so long' is the definition.
('adieu' can be a synonym of 'so long')
'gold-plated cube' is the wordplay.
'gold' becomes 'au' ('Au' can be a synonym of 'gold').
'plated' is an insertion indicator.
'cube' becomes 'die' (die is a kind of cube).
'au' placed around 'die' is 'ADIEU'.
'is' is the link.
(Other definitions for adieu that I've seen before include "Leave" , "I'm away (in France?)" , "cheers" , "Comment on leaving" , "Farewel" .)