Domain of British politician in Ireland (6)
I believe the answer is:
empire
'domain' is the definition.
(I know that empire is a type of domain)
'british politician in ireland' is the wordplay.
'british politician' becomes 'MP' (Member of Parliament).
'in' is an insertion indicator.
'ireland' becomes 'eire' (poetic name for Ireland).
'mp' placed inside 'eire' is 'EMPIRE'.
'of' acts as a link.
(Other definitions for empire that I've seen before include "Huge domain or group of companies" , "Great power" , "Large group of states" , "Former British-ruled countries" , "Corporation" .)