A number of children (6,4)
I believe the answer is:
cradle song
This could be a double definition.
'a number' is the first definition.
The definition and answer can be both related to communication as well as being singular nouns.
Perhaps you can see an association between them that I can't see?
'of children' is the second definition.
The definition suggests a singular noun which matches the answer.
Can you help me to learn more?
(Another definition for cradle song that I've seen is " Lullaby".)