Head old teaching union (3)
I believe the answer is:
nut
This clue is a double definition.
'head' is the first definition.
(informal term for somebody's head)
'old teaching union' is the second definition.
(National Union of Teachers, former teaching union)
(Other definitions for nut that I've seen before include "Hard-shelled fruit or informal eccentric" , "Vegetarian food" , "Headcase" , "Lover < or partner to bolt f" , "Enthusiastic person" .)