Stadium in region accommodating any number (5)
I believe the answer is:
arena
'stadium' is the definition.
(eg a sports arena)
'region accommodating any number' is the wordplay.
'region' becomes 'area' (area is a kind of region).
'accommodating' means one lot of letters goes inside another.
'any number' becomes 'n' (mathematical symbol for an unspecified number).
'area' placed around 'n' is 'ARENA'.
'in' is the link.
(Other definitions for arena that I've seen before include "Place of activity, especially conflict" , "Olympic venue?" , "Cheap Prom tickets" , "Area in centre of sports stadium" , "Sphere of action" .)