Thus written in musical notation (3)
I believe the answer is:
sic
'thus' is the definition.
('thus' in Latin)
'written in musical notation' is the wordplay.
'written in' indicates a hidden word.
'SIC' is hidden within 'musical notation'.
(Other definitions for sic that I've seen before include "'Thus, in printing (3)'" , "Thus in Corsica" , "Like" , "Thus - in Latin" , "It calls attention to a quoted mistake" .)