At least forty musical manuscripts (6)
I believe the answer is:
scores
I believe this clue is a double definition.
'at least forty' is the first definition.
('score' can mean 20, so 'scores' is at least 40)
'musical manuscripts' is the second definition.
(sheet music)
(Other definitions for scores that I've seen before include "Written forms of a musical composition" , "Scratches" , "Marks; is successful" , "Sets of twenty" , "Written forms of musical compositions" .)