Unknown river in old country (5)
I believe the answer is:
zaire
'old country' is the definition.
'unknown river' is the wordplay.
'unknown' becomes 'z' (algebra).
'river' becomes 'aire' (I've seen this before).
'z'+'aire'='ZAIRE'
'in' is the link.
(Other definitions for zaire that I've seen before include "Country, formerly" , "Central African country's former name" , "Former African republic" , "Previous name of Congo" , "Former name of DRC" .)