Gold coin - unknown but not special (8)
I believe the answer is:
ordinary
'not special' is the definition.
Both the definition and answer are adjectives. Perhaps you can see an association between them that I can't see?
'gold coin unknown' is the wordplay.
'gold' becomes 'or' ('or' is the heraldic name for gold).
'coin' becomes 'dinar' (monetary unit in some countries).
'unknown' becomes 'y' (algebra).
'or'+'dinar'+'y'='ORDINARY'
'but' acts as a link.
Can you help me to learn more?
(Other definitions for ordinary that I've seen before include "'Commonplace, banal (8)'" , "standard" , "Common" , "Common-or-garden" , "Undistinguished" .)