Ground in the Po valley (4)
I believe the answer is:
oval
'ground' is the definition.
(cricket ground)
'in the po valley' is the wordplay.
'in' indicates the answer is hidden within the clue.
'OVAL' is hidden within 'po valley'.
(Other definitions for oval that I've seen before include "6D 5A [TUBE STATION]" , "form of an egg" , "Where England regained the Ashes" , "Elliptical - cricket ground" , "Egg-shaped - cricket ground" .)