China, undecorated, round gold core's exterior (9)
I believe the answer is:
porcelain
'china' is the definition.
(I've seen this before)
'undecorated round gold core's exterior' is the wordplay.
'undecorated' becomes 'plain' (associated in meaning).
'round' is an insertion indicator.
'gold' becomes 'or' ('or' is the heraldic name for gold).
'exterior' suggests removing the centre.
'core' with its centre removed is 'ce'.
'or'+'ce'='orce'
'plain' going around 'orce' is 'PORCELAIN'.
(Other definitions for porcelain that I've seen before include "Fine white China" , "Form of ceramic" , "Jasper, perhaps" , "Translucent pottery" , "Fine ceramic ware" .)