Bird’s endless murmur (3)
I believe the answer is:
coo
'murmur' is the definition.
(cooing is a kind of murmuring)
'bird's endless' is the wordplay.
'bird' becomes 'coot' (coot is a kind of bird).
'endless' means to remove the last letter.
'coot' with its final letter removed is 'COO'.
(Other definitions for coo that I've seen before include "Gosh!" , "Murmur of a dove" , "crumbs" , "Expression of surprise (by a dove?)" , "Pigeon call" .)