Nothing alive ultimately in river (4)
I believe the answer is:
nile
'river' is the definition.
(Nile River is an example)
'nothing alive ultimately' is the wordplay.
'nothing' becomes 'nil' ('nil' can be a synonym of 'nothing').
'ultimately' says to take the final letters.
The last letter of 'alive' is 'e'.
'nil'+'e'='NILE'
'in' acts as a link.
(Other definitions for nile that I've seen before include "White or Blue river of Africa" , "Long river" , "Current" , "white flower?" , "River that's Blue and White" .)