River duck going to east (4)
I believe the answer is:
nile
'river' is the definition.
(Nile River is an example)
'duck going to east' is the wordplay.
'duck' becomes 'nil' (I've seen this before).
'going to' says to put letters next to each other.
'east' becomes 'e' ('E' can be a synonym of 'east').
'nil'+'e'='NILE'
(Other definitions for nile that I've seen before include "River that's Blue and White" , "Cairo's river" , "Green" , "World's longest river" , "Line (anag)" .)