Republic's revolutionary reign (5)
I believe the answer is:
niger
'republic's' is the definition.
(I've seen this before)
'revolutionary reign' is the wordplay.
'revolutionary' indicates an anagram.
'reign' anagrammed gives 'NIGER'.
(Other definitions for niger that I've seen before include "foreign banker" , "African country dominated by the Sahara, until 1969 a French colony" , "West African country, capital Niamey" , "Country W. of Chad" , "West African river and country" .)