The boss is a nut! (4)
I believe the answer is:
head
I believe this is a double definition.
'the boss' is the first definition.
(thesaurus)
'a nut' is the second definition.
(thesaurus)
'is' is the link.
(Other definitions for head that I've seen before include "Leader; stream source" , "Source of river, leader" , "Noggin" , "River or stream source" , "Edible part of cabbage" .)