Region around northern amphitheatre (5)
I believe the answer is:
arena
'amphitheatre' is the definition.
(I've seen this before)
'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' enclosing 'n' is 'ARENA'.
(Other definitions for arena that I've seen before include "Where gladiators fought" , "Place of activity, especially conflict" , "Central area in sports stadium" , "Where gladiatorial contests took place" , "Area of activity, boxing perhaps" .)