Slow passage with many a wrong note (5)
I believe the answer is:
lento
'slow passage' is the definition.
(I've seen this before)
'with many a wrong note' is the wordplay.
'with' is a charade indicator (letters next to each other).
'many' becomes 'l' ().
'a wrong' indicates an anagram.
'note' with letters rearranged gives 'ento'.
'l' put next to 'ento' is 'LENTO'.
(Other definitions for lento that I've seen before include "Musically low speed" , "Slow musically" , "How one scores slowly" , "Slow (mus.)" , "Scored slowly" .)