Warning initially given by teacher over waxy secretion (9)
I believe the answer is:
ambergris
'waxy secretion' is the definition.
(substance produced by sperm whales)
'warning initially given by teacher over' is the wordplay.
'warning' becomes 'amber' (amber traffic light).
'initially given by teacher over' becomes 'gris' (I am not sure about this - if you are sure you should believe this answer much more).
'amber'+'gris'='AMBERGRIS'
Can you help me to learn more?
(Other definitions for ambergris that I've seen before include "Wax-like substance" , "Sperm whale secretion used in perfume manufacture" , "Perfume ingredient from whales" , "substance floating in sea" , "A waxy substance" .)