Vivaldi wrote a lot of these, anything but priestly at the core (5)
I believe the answer is:
notes
'vivaldi wrote a lot' is the definition.
I can't tell whether this defines the answer.
'these anything but priestly at the core' is the wordplay.
'these 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 centre of 'definitearticle' is 'e'.
's' after 'e' is 'es'.
'not'+'es'='NOTES'
'of' is the link.
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" .)