Duck by English river (4)
I believe the answer is:
nile
'river' is the definition.
(I know that Nile is a river)
'duck by english' is the wordplay.
'duck' becomes 'nil' (both can mean zero).
'by' 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 "Current" , "'The longest river, blue and white (4)'" , "White or Blue river of Africa" , "Egypt's river" , "long-distance runner in Africa" .)