Crossword Genius

So left a note (3)

Ross

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" .)

I've seen this clue in the Evening Standard.
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