Gold, sweetheart, is a mineral! (3)
I believe the answer is:
'mineral' is the definition.
(ore is a kind of mineral)
'gold sweetheart' is the wordplay.
'gold' becomes 'or' ('or' is the heraldic name for gold).
'sweetheart' becomes 'e' (the heart/centre of 'sweet').
'or'+'e'='ORE'
'is a' acts as a link.
(Other definitions for ore that I've seen before include "Unwrought iron" , "Deposit of mine" , "Mineral or metal source" , "Danish currency" , "Naturally occurring mineral" .)