Rwandan king of old is back (5)
I believe the answer is:
tutsi
'rwandan' is the definition.
(Tutsis are an African ethnic group)
'king of old is back' is the wordplay.
'king of old' becomes 'Tut' (Tutankhamun).
'back' says the letters should be written backwards.
'is' written backwards gives 'si'.
'tut'+'si'='TUTSI'
(Other definitions for tutsi that I've seen before include "Member of a Bantu people of Rwanda and Burundi" , "Person from Burundi" , "member of a Bantu tribe" , "Member of a people living in Rwanda and Burundi" , "Rwanda tribe member" .)