Former country with a middle sized river! (5)
I believe the answer is:
zaire
'former country' is the definition.
'zaire' can be an answer for 'country' (Zaire is an example). I'm not certain of the 'former' bit.
'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'.
Can you help me to learn more?
(Other definitions for zaire that I've seen before include "Former African country name" , "Previous name of African state, achieved independence from Belgium in 1960" , "Central African country's former name" , "somewhere abroad, recalled?" , "renamed African area" .)