Contented cat's sound (4)
I believe the answer is:
'contented 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 "Cat's noise" , "Noise of a contented feline" , "Satisfaction from queen" , "Make contented sound like cat" , "Sound of a happy mog" .)