Existing head of monarchy's royal domain (5)
I believe the answer is:
realm
'royal domain' is the definition.
(I've seen this before)
'existing head of monarchy's' is the wordplay.
'existing' becomes 'real' (similar in meaning).
'head of' says to take the initial letters.
The first letter of 'monarchys' is 'm'.
'real'+'m'='REALM'
(Other definitions for realm that I've seen before include "Knowledge base" , "the UK, perhaps" , "World" , "Empire - state" , "Province" .)