Gold-plated carbon and copper exist (5)
I believe the answer is:
occur
'exist' is the definition.
(thesaurus)
'gold-plated carbon and copper' is the wordplay.
'gold' becomes 'or' ('or' is the heraldic name for gold).
'plated' is an insertion indicator.
'carbon' becomes 'c' (C is the chemical symbol for carbon).
'and' is a charade indicator (letters next to each other).
'copper' becomes 'cu' (chemical symbol).
'c'+'cu'='ccu'
'or' enclosing 'ccu' is 'OCCUR'.
(Other definitions for occur that I've seen before include "Go on" , "Spring to mind" , "appear" , "Befall" , "Happen - to come to mind" .)