Actual capital of mythical royal domain (5)
I believe the answer is:
realm
'royal domain' is the definition.
(I've seen this before)
'actual capital of mythical' is the wordplay.
'actual' becomes 'real' ('real' can be a synonym of 'actual').
'capital of' says to take the initial letters.
The initial letter of 'mythical' is 'm'.
'real'+'m'='REALM'
(Other definitions for realm that I've seen before include "Knowledge base" , "Empire - state" , "Kingdom could be lamer" , "Kingdom, or field of activity" , "Province" .)