Member in country realm (6)
I believe the answer is:
empire
'realm' is the definition.
(I know that empire is a type of realm)
'member in country' is the wordplay.
'member' becomes 'MP' (Member of Parliament).
'in' is an insertion indicator.
'country' becomes 'eire' (Eire is an example).
'mp' going into 'eire' is 'EMPIRE'.
(Other definitions for empire that I've seen before include "Group of countries ruled by single supreme authority" , "Over which 23 down rules" , "Widespread political or industrial organisation" , "Large organisation run from one point" , "Group of countries under one authority" .)