Crossword Genius

Each sound is a sound of contentment (4)

Ross

I believe the answer is:

purr

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'sound of contentment' is the definition.
(I've seen this before)

I don't understand the remainder of the clue.

Can you help me to learn more?

(Other definitions for purr that I've seen before include "Noise of a contented feline" , "Sound of cat (or engine)" , "Low vibratory sound" , "Appreciative feline sound" , "Cat's noise" .)

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