The note thus left (3)
I believe the answer is:
sol
'the note' is the definition.
(musical note as in do-re-mi-fa-sol-la-ti)
'thus left' is the wordplay.
'thus' becomes 'so' (synonyms).
'left' becomes 'l' (common abbreviation).
'so'+'l'='SOL'
(Other definitions for sol that I've seen before include "Note, old french coin" , "Roman counterpart of Helios" , "nearby star" , "Roman sun-god" , "The Sun (or Campbell?)" .)