One caring terribly about lack of knowledge (9)
I believe the answer is:
ignorance
'lack of knowledge' is the definition.
(I've seen this before)
'one caring terribly' is the wordplay.
'terribly' is an anagram indicator.
'one'+'caring'='onecaring'
'onecaring' is an anagram of 'IGNORANCE'.
'about' is the link.
(Other definitions for ignorance that I've seen before include "Lack of knowledge" , "bliss, they say" , "......... of the law is no excuse for breaking it" , "Stupidity" , "Nescience" .)