3rd century Gaelic bard and hero (6)
I believe the answer is:
ossian
(Other definitions for ossian that I've seen before include "Legendary Irish poet invented in eighteenth century; is a son (anag.)" , "Gaelic bard, son of Fingal" , "Fictitious Irish poet" , "Legendary Cthree Gaelic bard" , "Invented Irish hero" .)