Murmur softly (3)
I believe the answer is:
coo
'murmur softly' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for coo that I've seen before include "well I never!" , "Murmur of a dove" , "Soft sound of dove or baby" , "Murmur amorously" , "Dove's call" .)