Crossword Genius

Head teachers' brotherhood (3)

Ross

I believe the answer is:

nut

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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)'" .)

I've seen this clue in The Telegraph.
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