Murmur (3)
'murmur' is the definition.
(I know that susurrate is a more specific form of the action murmur)
This is all the clue.
'murmur' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for undertone that I've seen before include "Low or subdued voice pitch" , "Quiet way of speaking" , "Implicit meaning" .)
'murmur' is the definition.
(cooing is a kind of murmuring)
This is the entire clue.
(Other definitions for coo that I've seen before include "Soft sound of dove or baby" , "(Of pigeon) murmur" , "well I never!" , "Soft murmuring sound of a pigeon" , "Bill's companion" .)