The longest river in Africa (4)
I believe the answer is:
nile
'the longest river in africa' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for nile that I've seen before include "Green" , "'The longest river, blue and white (4)'" , "Egypt's river" , "Take a line to the river" , "Cairo's river" .)