Crackpot (3)
I believe the answer is:
nut
'crackpot' is the definition.
(I know that crackpot can be written as nut)
This is the entire clue.
(Other definitions for nut that I've seen before include "Mechanical fastener" , "Cashew, for example" , "Head (slang)" , "organized teachers" , "Hard edible seed" .)