Be at the root of crow calls, by the sound of it (5)
I believe the answer is:
cause
'sound of it' 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?
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for cause that I've seen before include "why some people fight" , "It brings about an effect" , "Make happen; something deserving support" , "Grounds" , "Producer of an effect" .)