Nothing seen on eastern river (4)
I believe the answer is:
nile
'river' is the definition.
(Nile River is an example)
'nothing seen on eastern' is the wordplay.
'nothing' becomes 'nil' (eg nil is zero in football).
'seen on' is a charade indicator (letters next to each other).
'eastern' becomes 'e'.
'nil'+'e'='NILE'
(Other definitions for nile that I've seen before include "World's longest river" , "long-distance runner in Africa" , "Egypt's river" , "Current" , "Cairo's river" .)