Egypt's river would be nothing without Egypt's capital (4)
I believe the answer is:
nile
'egypt's river' is the definition.
(I've seen this in another clue)
'nothing without egypt's capital' is the wordplay.
I cannot quite see how this works, but
'nothing' could be 'nil' (eg nil is zero in football) and 'nil' is found within the answer.
A single letter 'e' remains which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
'would be' is the link.
Can you help me to learn more?
(Other definitions for nile that I've seen before include "River associated with Cleopatra" , "River of eastern Africa" , "Egypt's river" , "Take a line to the river" , "Line (anag)" .)