Obsessive person in teachers’ union (3)
I believe the answer is:
nut
I believe this clue is a quadruple definition!!
'obsessive' is the first definition.
(I've seen this before)
'person' is the second definition.
(nut is a kind of person)
'in' acts as a link.
'teachers'' is the third definition.
(National Union of Teachers)
'union' is the fourth definition.
(National Union of Teachers)
(Other definitions for nut that I've seen before include "something turned when making fast > food" , "eg Almond, brazil" , "Hard shell of a locking device" , "Screwball" , "Fan" .)