Rule of politician in Irish Republic (6)
I believe the answer is:
empire
'rule' is the definition.
(thesaurus)
'politician in irish republic' is the wordplay.
'politician' becomes 'MP' (Member of Parliament).
'in' is an insertion indicator.
'irish republic' becomes 'eire' (synonyms).
'mp' placed into 'eire' is 'EMPIRE'.
'of' acts as a link.
(Other definitions for empire that I've seen before include "Group of states under the control one authority" , "Supreme power over many countries" , "The Romans, Byzantines and Russians all ruled one" , "Domain ruled by an emperor or empress" , "Of furniture and dress style of Napoleon I period" .)