Duck initially enters river (4)
I believe the answer is:
nile
'river' is the definition.
(I know that Nile is a river)
'duck initially enters' is the wordplay.
'duck' becomes 'nil' (both can mean zero).
'initially' says to take the initial letters.
The first letter of 'enters' is 'e'.
'nil'+'e'='NILE'
(Other definitions for nile that I've seen before include "Take a line to the river" , "White or Blue river of Africa" , "Line (anag)" , "Egypt's river" , "River (which is originally blue and white)" .)