Show old river (4)
I believe the answer is:
expo
'show' is the definition.
(short for exposition, a large exhibition)
'old river' is the wordplay.
'old' becomes 'ex' (prefix meaning former or onetime).
'river' becomes 'Po' (Italian river).
'ex'+'po'='EXPO'
(Other definitions for expo that I've seen before include "Show" , "Large international exhibition" , "Major international fair" , "Public exhibition" , "Public showing" .)