Stupidity is one caring stupidly (9)
I believe the answer is:
ignorance
'stupidity' is the definition.
(I've seen this before)
'one caring stupidly' is the wordplay.
'stupidly' is an anagram indicator (a stupid ordering of the letters).
'one'+'caring'='onecaring'
'onecaring' anagrammed gives 'IGNORANCE'.
'is' acts as a link.
(Other definitions for ignorance that I've seen before include "Lack of knowledge" , "Being in the dark" , "Stupidity" , "Lack of education" , "Groin acne (anag.)" .)