Any number banned from stadium environs (4)
I believe the answer is:
area
'environs' is the definition.
(environs means an area or region)
'any number banned from stadium' is the wordplay.
'any number' becomes 'n' (mathematical symbol for an unspecified number).
'banned from' indicates named letters should be taken away.
'stadium' becomes 'arena' (synonyms).
'arena' with 'n' removed is 'AREA'.
(Other definitions for area that I've seen before include "topic" , "Territory" , "environs" , "Field -- expanse" , "Region, space" .)