Dove's sound (3)
I believe the answer is:
coo
'dove's sound' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for coo that I've seen before include "Murmur lovingly" , "(Of pigeon) murmur" , "Soft sound of dove or baby" , "Bill's companion" , "Dove's call" .)