Fifth-century invader (3)
I believe the answer is:
hun
'fifth-century invader' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for hun that I've seen before include "One of fourth to fifth century invaders of Europe" , "Barbarian" , "Warlike nomad" , "old invader" , "One led by Attila" .)