Head teachers getting together (3)
I believe the answer is:
nut
I believe this clue is a double definition.
'head' is the first definition.
(informal term for somebody's head)
'teachers getting together' is the second definition.
(National Union of Teachers)
(Other definitions for nut that I've seen before include "Fanatical enthusiast" , "Cashew or hazel" , "Hard-shelled fruit or informal eccentric" , "Hard fruit or informal lunatic" , "keen admirer" .)