"Aida" river (4)
I believe the answer is:
nile
'aida river' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for nile that I've seen before include "World's longest river" , "Egypt's river" , "River of eastern Africa" , "River that's Blue and White" , "River (which is originally blue and white)" .)