Happy cat's sound (4)
I believe the answer is:
purr
'happy cat's sound' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for purr that I've seen before include "Sound of a happy mog" , "Cat noise" , "Make contented sound like cat" , "Low vibratory sound" , "Sound of a contented feline" .)