'Quiet, quiet!' originally stifled disorder (5)
I believe the answer is:
mumps
'disorder' is the definition.
(mumps is a kind of health problem)
'quiet quiet originally stifled' is the wordplay.
'quiet' becomes 'mum' (to keep mum is to stay quiet).
'quiet' becomes 'p' (music symbol for 'piano', ie soft or quiet).
'originally' suggests taking the first letters.
The first letter of 'stifled' is 's'.
'mum'+'p'+'s'='MUMPS'
(Other definitions for mumps that I've seen before include "Complaint" , "Infectious childhood disease" , "Disease once common in children" , "Acute viral disease, often in children" , "Swollen-face disease" .)