Head teachers' union? (3)
I believe the answer is:
nut
This clue is a triple definition!
'head' is the first definition.
(informal term for somebody's head)
'teachers'' is the second definition.
(National Union of Teachers)
'union?' is the third definition.
(National Union of Teachers)
(Other definitions for nut that I've seen before include "Hard-headed person" , "Fanatical enthusiast" , "Freak" , "Hard dry fruit - enthusiast" , "Somebody obsessed" .)