Region around northern stadium (5)
I believe the answer is:
arena
'stadium' is the definition.
('arena' can be a synonym of 'stadium')
'region around northern' is the wordplay.
'region' becomes 'area' ('area' can be a synonym of 'region'**).
'around' means one lot of letters goes inside another.
'northern' becomes 'n' (abbreviation e.g. N. Ireland).
'area' placed around 'n' is 'ARENA'.
(Other definitions for arena that I've seen before include "place for sporting contest" , "Place of public contest" , "sports ground" , "Area surrounded by seating, for sports, etc." , "Ring for contest" .)