Vivaldi wrote a lot of these, anything but priestly at the core (5)
I believe the answer is:
notes
'vivaldi wrote a lot of these' is the definition.
The definition suggests an adverb but the answer is not.
'anything but priestly at the core' is the wordplay.
'anything but' becomes 'not' (I've seen this in other clues).
'priestly' becomes 's' (I can't justify this - if you can you should give a lot more credence to this answer).
'at' means one lot of letters go next to another (I've seen this in other clues).
'the' becomes 'definite article' ('the' is the definite article in English).
'core' means to look at the middle letters.
The middle of 'definitearticle' is 'e'.
's' put after 'e' is 'es'.
'not'+'es'='NOTES'
Can you help me to learn more?
(Other definitions for notes that I've seen before include "Sees" , "Written comments" , "output from musicians" , "Memoranda" , "Cash" .)