Stupidity of one racing when drunk (9)
I believe the answer is:
ignorance
'stupidity' is the definition.
(I've seen this before)
'one racing when drunk' is the wordplay.
'when drunk' indicates an anagram (a drunken or disorderly form of the letters).
'one'+'racing'='oneracing'
'oneracing' is an anagram of 'IGNORANCE'.
'of' acts as a link.
(Other definitions for ignorance that I've seen before include "......... of the law is no excuse for breaking it" , "being uninformed" , "Groin acne (anag.)" , "Being in the dark" , "Nescience" .)