Region circling northern sphere of action (5)
I believe the answer is:
arena
'sphere of action' is the definition.
(I've seen this before)
'region circling northern' is the wordplay.
'region' becomes 'area' (synonyms).
'circling' means one lot of letters goes inside another (circling can mean going around or containing).
'northern' becomes 'n' (abbreviation e.g. N. Ireland).
'area' placed around 'n' is 'ARENA'.
(Other definitions for arena that I've seen before include "Circus ring" , "Area surrounded by seating, for sports, etc." , "Venue for contests" , "Playing area" , "Area in centre of sports stadium" .)