Left a note for the boy (3)
I believe the answer is:
lad
'the boy' is the definition.
(informal term for boy)
'left a note' is the wordplay.
'left' becomes 'L' (common abbreviation).
'note' becomes 'd' (musical note).
'l'+'a'+'d'='LAD'
'for' acts as a link.
(Other definitions for lad that I've seen before include "Youngster" , "Stable worker" , "Boy, young man" , "Son" , "(Housman's Shropshire) boy" .)