Stupid visionary sent up expansionist monarchy (7)
I believe the answer is:
assyria
'expansionist monarchy' is the definition.
Although both the answer and definition are singular nouns, I can't see how one could define the other.
'stupid visionary sent up' is the wordplay.
'stupid' becomes 'ass' (I am not sure about this - if you are sure you should believe this answer much more).
'visionary' becomes 'airy' (both can mean dream-like or illusory).
'sent up' says the letters should be written in reverse.
'airy' backwards is 'yria'.
'ass'+'yria'='ASSYRIA'
Can you help me to learn more?
(Other definitions for assyria that I've seen before include "Ancient Middle East kingdom - Ira says (anag)" , "old empire" , "Ancient kingdom of Mesopotamia" , "Ancient Semitic kingdom" , "Nineveh its capital" .)