Child's potty gone rusty (9)
I believe the answer is:
youngster
'child's' is the definition.
('youngster' can be a synonym of 'child')
'potty gone rusty' is the wordplay.
'potty' indicates anagramming the letters.
'gone'+'rusty'='gonerusty'
'gonerusty' with letters rearranged gives 'YOUNGSTER'.
(Other definitions for youngster that I've seen before include "Person of tender years" , "tyke" , "Tiddler" , "Kid" , "Juvenile" .)