Head teachers' brotherhood (3)
I believe the answer is:
nut
This could be a double definition.
'head' is the first definition.
(informal term for somebody's head)
'teachers' brotherhood' is the second definition.
Although both the answer and definition are singular nouns, I don't understand how they can define each other.
Can you help me to learn more?
(Other definitions for nut that I've seen before include "Butt" , "Seed with hard shell and firm kernel" , "One securing bolt" , "Fanatic (slang)" , "'Almond or cashew, say (3)'" .)