Region circling new sphere of action (5)
I believe the answer is:
arena
'sphere of action' is the definition.
(I've seen this before)
'region circling new' is the wordplay.
'region' becomes 'area' (area is a kind of region).
'circling' indicates putting letters inside (circling can mean going around or containing).
'new' becomes 'n' (common abbreviation eg NT for New Testament).
'area' going around 'n' is 'ARENA'.
(Other definitions for arena that I've seen before include "combat zone" , "Where gladiators fought" , "Scene of gladiatorial contest, perhaps" , "bowl? Possibly" , "Scene of action" .)