In Italy, that eastern river is so long! (7)
I believe the answer is:
cheerio
'is so long' is the definition.
(I've seen this before)
'in italy that eastern river' is the wordplay.
'in italy that' becomes 'che' ('that' in Italian).
'eastern' becomes 'e'.
'river' becomes 'rio' ('river' in Spanish**).
'che'+'e'+'rio'='CHEERIO'
(Other definitions for cheerio that I've seen before include "Ta-ta" , "Informal goodbye, sounds blithe" , "good health" , "Say this before drinking" , "Farewell - drinking toast" .)