Gains slid off sliding scales (9)
I believe the answer is:
glissandi
'sliding scales' is the definition.
The answer and definition can be both related to communication as well as being plural nouns.
Maybe they are linked in a way I don't understand?
'gains slid off' is the wordplay.
'off' indicates an anagram.
'gains'+'slid'='gainsslid'
'gainsslid' with letters rearranged gives 'GLISSANDI'.
Can you help me to learn more?
(Other definitions for glissandi that I've seen before include "Series of rapidly ascending or descending musical notes" , "Slides (music)" , "Musical terms" , "Players may perform them" , "showing smooth actions" .)