Contented sound as pussy unwinds, really relaxed initially (4)
I believe the answer is:
purr
'contented sound' is the definition.
(I've seen this before)
'pussy unwinds really relaxed initially' is the wordplay.
'initially' indicates taking the first letters.
The first letters of 'pussy unwinds really relaxed' is 'PURR'.
'as' acts as a link.
(Other definitions for purr that I've seen before include "content? I might" , "Sound of cat (or engine)" , "Express contentment" , "Sound of feline content" , "Noise of a contented feline" .)