Don's left the ass a note! (3)
I believe the answer is:
key
'a note' is the definition.
(I know that key is a type of note)
'don's left the ass' is the wordplay.
'left' suggests deleting specific letters.
'the ass' becomes 'donkey' (donkey is a kind of ass).
'donkey' with 'don' removed is 'KEY'.
(Other definitions for key that I've seen before include "explanation" , "locker" , "Scale system for music" , "Lock undoer" , "Device for lock" .)