Ring any number in region (5)
I believe the answer is:
arena
'ring' is the definition.
(thesaurus)
'any number in region' is the wordplay.
'any number' becomes 'n' (I've seen this in other clues).
'in' means one lot of letters goes inside another.
'region' becomes 'area' ('area' can be a synonym of 'region'**).
'n' inserted within 'area' is 'ARENA'.
(Other definitions for arena that I've seen before include "Place of great activity" , "Where gladiatorial contests took place" , "Contest venue" , "O2, for example" , "Field of combat" .)