Crackbrained (7)
I believe the answer is:
idiotic
'crackbrained' is the definition.
(I know that crackbrained can be written as idiotic)
This is the entire clue.
(Other definitions for idiotic that I've seen before include "Inviting ridicule" , "Witless" , "Ridiculous" , "Altogether nonsensical" , "Half witted" .)