A duck found on English river (4)
I believe the answer is:
nile
'river' is the definition.
(Nile River is an example)
'a duck found on english' is the wordplay.
'a duck' becomes 'nil' (I've seen this before).
'found on' says to put letters next to each other.
'english' becomes 'e' (abbreviation).
'nil'+'e'='NILE'
(Other definitions for nile that I've seen before include "White or Blue river of Africa" , "African river" , "Long river" , "Egyptian river" , "River (which is originally blue and white)" .)