A duck seen on English river (4)
I believe the answer is:
nile
'river' is the definition.
(Nile River is an example)
'a duck seen on english' is the wordplay.
'a duck' becomes 'nil' (I've seen this before).
'seen on' means one lot of letters go next to another.
'english' becomes 'e' (abbreviation).
'nil'+'e'='NILE'
(Other definitions for nile that I've seen before include "Egyptian river" , "River (which is originally blue and white)" , "long-distance runner in Africa" , "Cairo's river" , "White or Blue river of Africa" .)