Domain: actual kingdom, ultimately (5)
I believe the answer is:
realm
'domain' is the definition.
(I know that realm is a type of domain)
'actual kingdom ultimately' is the wordplay.
'actual' becomes 'real' (synonyms).
'ultimately' says to take the final letters.
The last letter of 'kingdom' is 'm'.
'real'+'m'='REALM'
(Other definitions for realm that I've seen before include "Country" , "Monarchy" , "where ruler holds sway?" , "field of study" , "Province" .)