Leaders never played violin until awful Nero? Throw that out - there are no supporting facts (8)
I believe the answer is:
unproven
'are no supporting facts' is the definition.
The answer and definition are different parts of speech. However, past participle verbs and adjectives sometimes define each other.
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for unproven that I've seen before include "Verdict" , "flat bread?" , "Not established as fact" , "Not demonstrated as true or factual" .)