Nothing seen by eastern river (4)
I believe the answer is:
nile
'river' is the definition.
(I know that Nile is a river)
'nothing seen by eastern' is the wordplay.
'nothing' becomes 'nil' (eg nil is zero in football).
'seen by' means one lot of letters go next to another.
'eastern' becomes 'e'.
'nil'+'e'='NILE'
(Other definitions for nile that I've seen before include "World's longest river" , "African river" , "long-distance runner in Africa" , "Green" , "Cairo's river" .)