Nation amazingly large one, advanced (7)
I believe the answer is:
algeria
'nation' is the definition.
(Algeria is an example)
'amazingly large one advanced' is the wordplay.
'amazingly' is an anagram indicator.
'one' becomes 'i' (Roman numeral).
'advanced' becomes 'a' ().
'large' with letters rearranged gives 'alger'.
'alger'+'i'+'a'='ALGERIA'
(Other definitions for algeria that I've seen before include "Land" , "Strange regalia of a North African state" , "country in Africa" , "North African country" , "North-west African country" .)