Sound made by a pigeon (3)
I believe the answer is:
coo
'sound made by a pigeon' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for coo that I've seen before include "Dove-like noise" , "Murmur, speak softly" , "well I never!" , "Avian sound" , "Murmur amorously" .)