Old power in America is surprisingly regressive (7)
I believe the answer is:
prussia
'old power' is the definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.
'in america is surprisingly regressive' is the wordplay.
'in' indicates the answer is hidden within the clue.
'regressive' shows that the letters should be reversed in order (to regress is to go backwards).
'aissurp' is hidden within 'america is surprisingly'.
'aissurp' written backwards gives 'PRUSSIA'.
Can you help me to learn more?
(Other definitions for prussia that I've seen before include "It fought France, Eighteen-seventy" , "Bismarck its PM" , "Old European state" , "Powerful pre-1871 state of central Europe" , "Former German state" .)