Desperado finally banished from district backing African river (5)
I believe the answer is:
'african river' is the definition.
(I've seen this before)
I can't explain the rest of the clue.
(Other definitions for niger that I've seen before include "African country dominated by the Sahara, until 1969 a French colony" , "River of West Africa" , "Major West African river" , "foreign banker" , "one flowing through Mali" .)