Insult notes on celebrity (6)
I believe the answer is:
defame
'insult' is the definition.
(thesaurus)
'notes on celebrity' is the wordplay.
'notes' becomes 'de' (two musical notes).
'on' says to put letters next to each other (in a down clue, letters appear on others).
'celebrity' becomes 'fame' (celebrity can mean the quality of being famous).
'de'+'fame'='DEFAME'
(Other definitions for defame that I've seen before include "to pretetrate slander?" , "Traduce" , "Smear" , "Run down" , "'Denigrate, slander (6)'" .)