One intervenes in disease after singular stupidity (9)
I believe the answer is:
silliness
'stupidity' is the definition.
(I've seen this before)
'one intervenes in disease after singular' is the wordplay.
'one' becomes 'i' (Roman numeral).
'intervenes in' indicates putting letters inside.
'disease' becomes 'illness' (illness is a kind of health problem).
'after' is a charade indicator (letters next to each other).
'singular' becomes 's' (abbreviation sometimes used in dictionaries).
'i' going within 'illness' is 'illiness'.
'illiness' put after 's' is 'SILLINESS'.
(Other definitions for silliness that I've seen before include "Folly" , "stupidity" , "Foolish quality" .)