Amateur plays Nero in disastrous fashion (9)
I believe the answer is:
layperson
'amateur' is the definition.
(I've seen this before)
'plays nero in disastrous fashion' is the wordplay.
'in disastrous fashion' indicates an anagram (a disastrous spelling of the word).
'plays'+'nero'='playsnero'
'playsnero' anagrammed gives 'LAYPERSON'.
(Other definitions for layperson that I've seen before include "Amateur" , "one of his flock maybe" , "parishioner?" , "Non-specialist" , "Not an expert" .)