Slow, musically (6)
'slow musically' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for lento that I've seen before include "(Of tempo) slow" , "Scored slowly" , "Slowly (mus.)" , "It's slower than adagio" , "Slow (mus.)" .)
(Other definitions for andante that I've seen before include "Slow, even time (mus.)" , "Moderately slow musical passage" , "For players, rather slow" , "not quickly" , "Musical instruction" .)