Assembly incorporates Northern Ireland province (7)
I believe the answer is:
munster
'ireland province' is the definition.
'assembly incorporates northern' is the wordplay.
'assembly' becomes 'muster'.
'incorporates' is an insertion indicator.
'northern' becomes 'n' (abbreviation e.g. N. Ireland).
'muster' enclosing 'n' is 'MUNSTER'.
(Other definitions for munster that I've seen before include "Province of Ireland" , "European city" , "Turns me into a province" , "Irish province or German city" , "Irish quarter?" .)