Screwball (8)
I believe the answer is:
crackpot
'screwball' is the definition.
(I know that screwball can be written as crackpot)
This is all the clue.
(Other definitions for crackpot that I've seen before include "crazy type" , "Eccentric (colloq.)" , "Bats" , "Mad" , "Idiot" .)