Kitty and Tom are pleased to do it (4)
I believe the answer is:
purr
'it' is the definition.
The definition suggests a singular noun which matches the answer.
I can't explain the rest of the clue.
Can you help me to learn more?
(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" .)