Dove's cry (3)
I believe the answer is:
'dove's cry' 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 "crumbs" , "Call like a dove" , "Dove's call" , "Make loving sounds -bill and . . ." , "Murmur of a dove" .)