Thus left a note (3)
I believe the answer is:
sol
'a note' is the definition.
(I've seen this before)
'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 "Spanish and poetic sun" , "The Sun (or Campbell?)" , "Fifth note of scale" , "Note, old french coin" , "The sun in Spain" .)