Royal domain in actual miles (5)
I believe the answer is:
realm
'royal domain' is the definition.
(realm can mean a kingdom)
'actual miles' is the wordplay.
'actual' becomes 'real' (synonyms).
'miles' becomes 'm' (common abbreviation).
'real'+'m'='REALM'
'in' is the link.
(Other definitions for realm that I've seen before include "Kingdom; field of interest" , "Area of knowledge under discussion" , "Area of interest/activity" , "Knowledge base" , "Area of influence" .)