Quiet sound of pain in body cavity (5)
I believe the answer is:
pouch
'body cavity' is the definition.
(pouch is a kind of body cavity)
'quiet sound of pain' is the wordplay.
'quiet' becomes 'p' (music symbol for 'piano').
'sound of pain' becomes 'ouch'.
'p'+'ouch'='POUCH'
'in' acts as a link.
(Other definitions for pouch that I've seen before include "Bag" , "Marsupial's baby carrier" , "Kangaroo's pocket" , "Container for tobacco or young" , "joey's home" .)