Actual leader of monarchy in kingdom (5)
I believe the answer is:
realm
'kingdom' is the definition.
(thesaurus)
'actual leader of monarchy' is the wordplay.
'actual' becomes 'real' ('real' can be a synonym of 'actual').
'leader of' says to take the initial letters.
The initial letter of 'monarchy' is 'm'.
'real'+'m'='REALM'
'in' is the link.
(Other definitions for realm that I've seen before include "Country" , "Area of influence" , "Kingdom could be lamer" , "Knowledge base" , "Kingdom; field of interest" .)