Murmured sound is not entirely unenthusiastic (3)
I believe the answer is:
coo
'murmured sound' is the definition.
'not entirely unenthusiastic' is the wordplay.
'not entirely' means to remove the last letter.
'unenthusiastic' becomes 'cool' (I've seen this before).
'cool' with its last letter removed is 'COO'.
'is' is the link.
(Other definitions for coo that I've seen before include "Goodness me!" , "Gosh!" , "Murmur lovingly" , "Avian sound" , "Soft sound of dove or baby" .)