Supreme power of politician in Ireland (6)
I believe the answer is:
empire
'supreme power' is the definition.
(I've seen this before)
'politician in ireland' is the wordplay.
'politician' becomes 'mp'.
'in' indicates putting letters inside.
'ireland' becomes 'eire' ('Eire' can be a synonym of 'ireland').
'mp' put into 'eire' is 'EMPIRE'.
'of' acts as a link.
(Other definitions for empire that I've seen before include "Multinational" , "Group of countries ruled by single supreme authority" , "In state of New York" , "Wide dominion" , "Large organisation run from one point" .)