Crib sound
I believe the answer is:
coo
'crib sound' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for coo that I've seen before include "Sound like a dove" , "(Of pigeon) murmur" , "Murmur lovingly" , "Dove's noise" , "Expression of surprise (by a dove?)" .)