Kingdom in actual miles (5)
I believe the answer is:
realm
'kingdom' is the definition.
(thesaurus)
'actual miles' is the wordplay.
'actual' becomes 'real' ('real' can be a synonym of 'actual').
'miles' becomes 'm' (common abbreviation).
'real'+'m'='REALM'
'in' acts as a link.
(Other definitions for realm that I've seen before include "World" , "Kingdom's" , "the UK, perhaps" , "Kingdom or sphere" , "Area of interest/activity" .)