A good thing for seedy teachers? (3)
I believe the answer is:
nut
'teachers?' is the definition.
(National Union of Teachers)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for nut that I've seen before include "Somebody obsessed" , "Large, hard-shelled seed" , "Hazel; beech" , "needing to get tight?" , "Head (sl.)" .)