Richard set out children's disease (7)
I believe the answer is:
rickets
'children's disease' is the definition.
(I've seen this before)
'richard set out' is the wordplay.
'richard' becomes 'rick' (common nickname for Richard).
'out' indicates anagramming the letters (out can mean wrong or inaccurate).
'set' anagrammed gives 'ets'.
'rick'+'ets'='RICKETS'
(Other definitions for rickets that I've seen before include "Childhood disease caused by a deficiency of vitamin D" , "Children's disease" , "Bone-distorting disease" , "Bone disease, caused by vitamin D deficiency" .)