District Officer originally involved in the rise of Niger (6)
I believe the answer is:
region
'district' is the definition.
(I've seen this before)
'officer originally involved in the rise of niger' is the wordplay.
'originally' says to take the initial letters.
'involved in' is an insertion indicator.
'the rise of' says the letters should be written in reverse.
The first letter of 'officer' is 'o'.
'niger' back-to-front is 'regin'.
'o' inserted within 'regin' is 'REGION'.
(Other definitions for region that I've seen before include "Kind of territory to ignore" , "Area,territory" , "Wide area" , "Tract or area" , "Part of a country" .)