Speak ill of brilliant ace, the writer (6)
I believe the answer is:
defame
'speak ill of' is the definition.
(to defame someone is to speak ill of them)
'brilliant ace the writer' is the wordplay.
'brilliant' becomes 'def' (def can informally mean excellent).
'ace' becomes 'a'.
'the writer' becomes 'me' (the person who wrote the clues).
'def'+'a'+'me'='DEFAME'
(Other definitions for defame that I've seen before include "'Denigrate, slander (6)'" , "Traduce" , "to pretetrate slander?" , "Attack someone's good name" , "Attack good reputation, speak ill of" .)