Region holding northern sports venue (5)
I believe the answer is:
arena
'sports venue' is the definition.
(I've seen this before)
'region holding northern' is the wordplay.
'region' becomes 'area' (synonyms).
'holding' 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 public events" , "battlefieId" , "Engagement ring" , "Sit around here being entertained" , "O2, for example" .)