One caring about lack of knowledge (9)
I believe the answer is:
ignorance
'lack of knowledge' is the definition.
(I've seen this in another clue)
'one caring about' is the wordplay.
'about' indicates an anagram.
'one'+'caring'='onecaring'
'onecaring' is an anagram of 'IGNORANCE'.
(Other definitions for ignorance that I've seen before include "lack of awareness" , "Bliss perhaps" , "bliss, they say" , "......... of the law is no excuse for breaking it" , "Nescience" .)