Old ladies switched venue for sporting event (4)
I believe the answer is:
oval
'venue for sporting event' is the definition.
(London cricket ground)
'old ladies switched' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'ladies' becomes 'lav' (the ladies' lavatory).
'switched' says the letters should be written backwards.
'lav' written backwards gives 'val'.
'o'+'val'='OVAL'
(Other definitions for oval that I've seen before include "Surrey county cricket ground" , "Ellipse" , "Geometric shape" , "34 [CRICKETER] 's domain" , "Stretched circle?" .)