Impressive American building (7)
I believe the answer is:
rotunda
This clue could be a double definition.
'impressive american' is the first definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.
'building' is the second definition.
(rotunda is a kind of building)
Can you help me to learn more?
(Other definitions for rotunda that I've seen before include "Round domed building" , "Round domed room" , "Do a turn (anag) -- circular domed building" , "Round building or hall" , "Circular room or building" .)