Site of no architectural merit from around 21 BC (9)
I believe the answer is:
carbuncle
'site of no' is the definition.
The answer and definition can be both substances as well as being singular nouns.
Perhaps you can see an association between them that I can't see?
'architectural merit from around 21 bc' is the wordplay.
I cannot really see how this works, but
'around' could be 'ca' (circa) and 'ca' is found in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for carbuncle that I've seen before include "stone" , "eyesore architecturally" , "Cluster of boils - jewel" , "cheaper jewel?" , "Red gemstone; sort of boil" .)