Somehow a song slid off sliding scale (9)
I believe the answer is:
glissando
'off sliding scale' is the definition.
The definition and answer can be both related to communication as well as being singular nouns.
Perhaps they are linked in a way I don't understand?
'somehow a song slid' is the wordplay.
'somehow' indicates anagramming the letters.
'a'+'song'+'slid'='asongslid'
'asongslid' anagrammed gives 'GLISSANDO'.
Can you help me to learn more?
(Other definitions for glissando that I've seen before include "trombone slide, for example" , "Slide between two notes" , "Sliding series of notes (mus.)" , "Musical flourish" , "Re-interpreting this musical direction" .)