Sort of bun devoured but not defeated (8)
I believe the answer is:
'not defeated' is the definition.
(I've seen this before)
'sort of bun devoured' is the wordplay.
'sort of' indicates anagramming the letters.
'devoured' becomes 'eaten' (similar in meaning).
'bun' with letters rearranged gives 'unb'.
'but' is the link.
(Other definitions for unbeaten that I've seen before include "having at worst drawn badly?", "Yet to lose", "Not yet defeated", "Victorious", "second to none".)