Pigeon's murmur (3)
I believe the answer is:
coo
'pigeon'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 "Breathe" , "Sound coming from dovecote" , "Pigeon's sound" , "Sound made by doves" , "Murmur lovingly" .)