Genuine error that’s serious by reported visionary (7)
I believe the answer is:
sincere
'genuine' is the definition.
(sincere can mean authentic or genuine)
'error that's serious by reported visionary' is the wordplay.
'error' becomes 'sin' (a sin is a moral error).
'that's serious by reported visionary' becomes 'cere' (I can't explain this - if you can you should give a lot more credence to this answer).
'sin'+'cere'='SINCERE'
Can you help me to learn more?
(Other definitions for sincere that I've seen before include "Not hypocritical" , "straightforward" , "(Of person) genuine" , "Pure" , "Free from deception" .)