One racing about in incomprehension (9)
I believe the answer is:
ignorance
'incomprehension' is the definition.
The definition and answer can be both related to thinking as well as being singular nouns.
Perhaps you can see a link between them that I can't see?
'one racing about' is the wordplay.
'about' indicates an anagram.
'one'+'racing'='oneracing'
'oneracing' anagrammed gives 'IGNORANCE'.
'in' acts as a link.
Can you help me to learn more?
(Other definitions for ignorance that I've seen before include "lack of awareness" , "bliss, they say" , "may be considered bliss" , "Being in the dark" , "Not knowing" .)