Speak sweetly (3)
I believe the answer is:
coo
'speak sweetly' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for coo that I've seen before include "Dove-like noise" , "(Of pigeon) murmur" , "Bill's companion" , "well I never!" , "Murmur amorously" .)