Fifth-century barbarian (3)
I believe the answer is:
hun
'fifth-century barbarian' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for hun that I've seen before include "One led by Attila" , "Barbarian" , "Attila was a great one" , "Warlike invader" , "Ancient invader" .)