Duck seen on English river (4)
I believe the answer is:
nile
'river' is the definition.
(Nile River is an example)
'duck seen on english' is the wordplay.
'duck' becomes 'nil' (I've seen this before).
'seen on' is a charade indicator (letters next to each other).
'english' becomes 'e' (abbreviation).
'nil'+'e'='NILE'
(Other definitions for nile that I've seen before include "River at Luxor" , "River (which is originally blue and white)" , "'The longest river, blue and white (4)'" , "World's longest river" , "River of eastern Africa" .)