MP in Ireland for big business (6)
I believe the answer is:
empire
'big business' is the definition.
(I've seen this in another clue)
'mp in ireland' is the wordplay.
'in' is an insertion indicator.
'ireland' becomes 'eire' (poetic name for Ireland).
'mp' going inside 'eire' is 'EMPIRE'.
'for' acts as a link.
(Other definitions for empire that I've seen before include "Former British rule" , "Corporation" , "In state of New York" , "Several states ruled over by a single monarch" , "Sovereign state" .)