Kitty and Tom are pleased to do it (4)

I believe the answer is:

purr

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'it' is the definition.
The definition suggests a singular noun which matches the answer.

I can't explain the rest of the clue.

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(Other definitions for purr that I've seen before include "Cat's coo" , "Sound of a happy mog" , "Soft sound from car or cat" , "Low vibratory sound" , "Sound of feline content" .)

I've seen this clue in The Globe and Mail.

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