Sully notes on dossier (6)
I believe the answer is:
defile
'sully' is the definition.
(I know that sully can be written as defile)
'notes on dossier' is the wordplay.
'notes' becomes 'de' (two musical notes).
'on' means one lot of letters go next to another.
'dossier' becomes 'file' (a dossier is a file or set of information).
'de'+'file'='DEFILE'
(Other definitions for defile that I've seen before include "Spoil; narrow gorge" , "Contaminate" , "Fed lie to pollute" , "Spoil; steep-sided gorge" , "Narrow pass; pollute" .)