There's many a kingdom within his exceptional reach (8)
I believe the answer is:
archduke
'within his exceptional reach' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'there's many a kingdom' is the wordplay.
I cannot quite see how this works, but
'many' could be 'd' (Roman numeral for 500) and 'd' is located in the answer.
'a' is present in the answer.
'kingdom' could be 'uk' (UK is an example) and 'uk' is found in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for archduke that I've seen before include "Emperor's son" , "Austrian noble" , "Top nobleman" , "noted trio" , "European nobleman" .)