Duck on English river (4)
I believe the answer is:
nile
'river' is the definition.
(I know that Nile is a river)
'duck on english' is the wordplay.
'duck' becomes 'nil' (both can mean zero).
'on' means one lot of letters go next to another (in a down clue, letters appear on others).
'english' becomes 'E' (abbreviation).
'nil'+'e'='NILE'
(Other definitions for nile that I've seen before include "White or Blue river of Africa" , "Cairo's river" , "River associated with Cleopatra" , "Line (anag)" , "African river" .)