Region encompassing northern battleground (5)
I believe the answer is:
arena
'battleground' is the definition.
(I've seen this before)
'region encompassing northern' is the wordplay.
'region' becomes 'area' ('area' can be a synonym of 'region'**).
'encompassing' 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 of activity, especially conflict" , "Place of combat" , "Sit around here being entertained" , "Sporting venue enclosed by seats" , "Gladiatorial venue" .)