Childhood ailment (5)
I believe the answer is:
mumps
'childhood ailment' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for mumps that I've seen before include "Viral disease mainly affecting children" , "Acute viral disease, often in children" , "Viral disease, dangerous to adult males" , "Swollen-face disease" , "harmful disease" .)