Pigeon's sound (3)
I believe the answer is:
coo
'pigeon'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 "Soft murmuring sound of a pigeon" , "Expression of surprise (by a dove?)" , "Surprised expression" , "Pigeon's sound" , "Bill's companion" .)