Dove's murmur (3)
I believe the answer is:
coo
'dove's murmur' 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 "Soft sound of dove or baby" , "Murmur, speak softly" , "(Of pigeon) murmur" , "Sound coming from dovecote" , "Pigeon noise" .)