Diamond found in C&A party (9)
I believe the answer is:
carbonado
'diamond' is the definition.
(I know that carbonado is a type of diamond)
'ca party' is the wordplay.
'ca' becomes 'carbona' (I can't justify this - if you can you should give a lot more credence to this answer).
'party' becomes 'do' (do is a kind of party).
'carbona'+'do'='CARBONADO'
'found in' is the link.
Can you help me to learn more?
(Other definitions for carbonado that I've seen before include "Dark diamond used in industry for drilling and polishing" , "Meat grilled" , "Beef stew; black diamond" .)