It's noted for its slowness (5)
I believe the answer is:
largo
'slowness' is the definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for largo that I've seen before include "Florida key: musical slow movement" , "Musically in slow tempo and dignified style" , "Slow and dignified tempo in music" , "Played with dignified slowness" , "Slow and dignified, like Handel's composition" .)