Section in stadium not new (4)
I believe the answer is:
area
'section' is the definition.
(I've seen this in another clue)
'stadium not new' is the wordplay.
'stadium' becomes 'arena' (synonyms).
'not' is a deletion indicator.
'new' becomes 'n' (common abbreviation eg NT for New Testament).
'arena' with 'n' taken away is 'AREA'.
'in' is the link.
(Other definitions for area that I've seen before include "The measure of a surface" , "Speaking formally to (someone)" , "Succession, order" , "Space occupied by something" , "Region, zone" .)