Head is crazy person (3)
I believe the answer is:
nut
I believe this is a double definition.
'head' is the first definition.
(informal term for somebody's head)
'crazy person' is the second definition.
(nut can informally mean an insane person)
'is' is the link.
(Other definitions for nut that I've seen before include "Bolt's partner" , "Hard edible seed" , "Bolt counterpart" , "Screwball" , "Seed with hard shell and firm kernel" .)