Wayward son is a legendary Gaelic poet (6)
I believe the answer is:
ossian
'legendary gaelic poet' is the definition.
(I've seen this before)
'wayward son is a' is the wordplay.
'wayward' is an anagram indicator (wayward means out of control or chaotic).
'son'+'is'+'a'='sonisa'
'sonisa' is an anagram of 'OSSIAN'.
(Other definitions for ossian that I've seen before include "Gaelic bard, son of Fingal" , "Fictitious Irish poet" , "Legendary Irish poet invented in eighteenth century; is a son (anag.)" , "Invented Irish hero" , "Legendary Cthree Gaelic bard" .)