So left a note (3)
I believe the answer is:
sol
'a note' is the definition.
(musical note as in do-re-mi-fa-sol-la-ti)
'so left' is the wordplay.
'left' becomes 'l' (common abbreviation).
'so'+'l'='SOL'
(Other definitions for sol that I've seen before include "The Sun (or Campbell?)" , "Roman sun-god" , "Peruvian currency; Roman sun god" , "Note; Peruvian currency" , "Musical note" .)