Actual number in the kingdom (5)
I believe the answer is:
realm
'kingdom' is the definition.
(thesaurus)
'actual number' is the wordplay.
'actual' becomes 'real' ('real' can be a synonym of 'actual').
'number' becomes 'm' (M is an example).
'real'+'m'='REALM'
'in the' acts as a link.
(Other definitions for realm that I've seen before include "Kingdom's" , "where ruler holds sway?" , "Area of knowledge under discussion" , "Kingdom could be lamer" , "World" .)