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' says the answer is hidden in the clue.
'SIC' can be found hidden inside '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" .)