Region in stadium not new (4)
I believe the answer is:
area
'region' is the definition.
(I know that area is a type of region)
'stadium not new' is the wordplay.
'stadium' becomes 'arean' (I've seen this before).
'not' indicates named letters should be taken away.
'new' becomes 'n' (common abbreviation eg NT for New Testament).
'arean' with 'n' removed is 'AREA'.
'in' acts as a link.
(Other definitions for area that I've seen before include "land" , "District; plot" , "Region, space" , "District; locale" , "Length multiplied by breadth" .)