Head teachers, once obsessive (3)
I believe the answer is:
nut
I believe this is a triple definition!
'head' is the first definition.
(informal term for somebody's head)
'teachers' is the second definition.
(National Union of Teachers)
'once' is a link.
'obsessive' is the third definition.
(informal term for a fan or aficionado)
(Other definitions for nut that I've seen before include "Hard shell of a locking device" , "One securing bolt" , "'Almond or cashew, say (3)'" , "Fanatical enthusiast" , "Hard kernel of minutiae" .)