Nothing to the east but a river (4)
I believe the answer is:
nile
'a river' is the definition.
(I know that Nile is a river)
'nothing to the east' is the wordplay.
'nothing' becomes 'nil' (eg nil is zero in football).
'to the east' becomes 'E' ('E' can be a synonym of 'east').
'nil'+'e'='NILE'
'but' is the link.
(Other definitions for nile that I've seen before include "White or Blue river of Africa" , "Take a line to the river" , "River that's Blue and White" , "World's longest river" , "River (which is originally blue and white)" .)