Food specialist's threatening note to an artist? (9)
I believe the answer is:
dietitian
'food specialist's' is the definition.
(expert in diet)
'threatening note to an artist?' is the wordplay.
'threatening note' becomes 'die' (I can't explain this - if you can you should believe this answer much more).
'to an artist?' becomes 'Titian' (Titian is an example).
'die'+'titian'='DIETITIAN'
(Other definitions for dietitian that I've seen before include "Medical adviser" , "Food specialist" , "Food guru" , "one who regulates courses" , "Initiated for food expert" .)