As opposed to that French piece the dealer's priced highly (7)
I believe the answer is:
antique
'as opposed to that french piece the dealer's priced highly' is the definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.
'as opposed to that french' is the wordplay.
'as opposed' becomes 'anti' (similar in meaning. I am not sure about the 'as' bit.).
'to that french' becomes 'que' (French for 'that').
'anti'+'que'='ANTIQUE'
Can you help me to learn more?
(Other definitions for antique that I've seen before include "Archaic" , "Valuable due to age, like some furniture" , "Valuable old object" , "(Something) valuable because of age and quality" , "Object from the past sought by collectors" .)