African country with a middle-sized river (5)
I believe the answer is:
zaire
'african country' is the definition.
(Zaire is an example)
'with a middle-sized river' is the wordplay.
'with' means one lot of letters go next to another.
'a middle' means to look at the middle letters.
'river' becomes 'aire' (I've seen this before**).
The middle of 'sized' is 'z'.
'z'+'aire' is 'ZAIRE'.
(Other definitions for zaire that I've seen before include "Old name for African republic" , "Democratic Republic of the Congo, formerly" , "Central African country's former name" , "Democratic Republic of Congo's former name" , "Former name of DRC" .)