One of Africa's largest cities (5)
I believe the answer is:
cairo
'one of africa's largest cities' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for cairo that I've seen before include "African city" , "Egyptian capital" , "Al Qahira" , "Port upstream from the Nile delta" , "Capital city on the Nile" .)