A bad act by one twerp, the ultimate in crazy folly (9)
I believe the answer is:
asininity
'folly' is the definition.
(thesaurus)
'a bad act by one twerp the ultimate in crazy' is the wordplay.
'bad act' becomes 'sin'.
'by' says to put letters next to each other.
'one' becomes 'i' (Roman numeral).
'twerp' becomes 'nit' (both can mean an annoying person).
'the ultimate in' suggests the final letters.
The last letter of 'crazy' is 'y'.
'a'+'sin'+'i'+'nit'+'y'='ASININITY'
(Other definitions for asininity that I've seen before include "Idiocy" , "Foolishness as of donkey" , "Extreme foolishness" , "Stupidity" .)