Slowly, in music (5)
(Other definitions for adagio that I've seen before include "going slowly in work" , "Movement in slow time" , "Slow movement in musical work" , "In slow time (mus.)" , "Slowly (mus.)" .)
'slowly in music' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for lento that I've seen before include "How one scores slowly" , "Scored slowly" , "(When playing music) slowly" , "It's slower than adagio" , "Slow (mus.)" .)
(Other definitions for largo that I've seen before include "Handel's piece?" , "Slow and dignified, like Handel's composition" , "In a slow tempo (mus.)" , "In a stately manner, musically" , "Musical piece" .)