Unknown number leaving stadium in neighbourhood (4)
I believe the answer is:
area
'neighbourhood' is the definition.
(a neighbourhood is an urban area)
'unknown number leaving stadium' is the wordplay.
'unknown number' becomes 'n'.
'leaving' indicates named letters should be taken away.
'stadium' becomes 'arean' (I've seen this before).
'arean' with 'n' taken away is 'AREA'.
'in' is the link.
(Other definitions for area that I've seen before include "Region; extent of a surface" , "Patch" , "'Tract, region (4)'" , "topic" , "Field -- expanse" .)