Any number leaving stadium locality (4)
I believe the answer is:
area
'locality' is the definition.
(I've seen this before)
'any number leaving stadium' is the wordplay.
'any number' becomes 'n' (mathematical symbol for an unspecified number).
'leaving' is a deletion indicator.
'stadium' becomes 'arean' (I've seen this before).
'arean' with 'n' taken away is 'AREA'.
(Other definitions for area that I've seen before include "environs" , "District; plot" , "land" , "Locality" , "Space occupied by something" .)