Head teachers? (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' is the second definition.
(National Union of Teachers)
(Other definitions for nut that I've seen before include "Freak" , "Somebody obsessed" , "Lover < or partner to bolt f" , "Hard-skinned fruit" , "Brazil, for instance" .)