Murmuring voice (3)
I believe the answer is:
coo
'murmuring voice' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for coo that I've seen before include "Expression of surprise (by a dove?)" , "Sound like a dove" , "Goodness me!" , "Make loving sounds -bill and . . ." , "Gosh!" .)