River duck approaching east (4)
I believe the answer is:
nile
'river' is the definition.
(Nile River is an example)
'duck approaching east' is the wordplay.
'duck' becomes 'nil' (I've seen this before).
'approaching' 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 "Take a line to the river" , "Cleopatra's river" , "River of Egypt" , "River associated with Cleopatra" , "Green" .)