Old African country with a middle-sized river (5)
I believe the answer is:
zaire
'old african country' is the definition.
'zaire' can be an answer for 'country' (Zaire is an example). I'm not certain of the 'old african' bit.
'with a middle-sized river' is the wordplay.
'with' says to put letters next to each other.
'a middle' indicates the central letters.
'river' becomes 'aire' (River Aire in Yorkshire).
The centre of 'sized' is 'z'.
'z' next to 'aire' is 'ZAIRE'.
Can you help me to learn more?
(Other definitions for zaire that I've seen before include "Democratic Republic of the Congo, formerly" , "Country, formerly" , "Democratic Republic of Congo's former name" , "somewhere abroad, recalled?" , "Former African republic" .)