I poop on teacher, old fossil? (9)
I believe the answer is:
iguanodon
'old fossil?' is the definition.
(type of dinosaur)
'i poop on teacher' is the wordplay.
'poop' becomes 'guano' (bird excrement used as fertiliser).
'on' says to put letters next to each other.
'teacher' becomes 'don' (don is a kind of teacher).
'i'+'guano'+'don'='IGUANODON'
(Other definitions for iguanodon that I've seen before include "Old lizard" , "reptile now extinct" , "Massive dinosaur of the early Cretaceous period" , "fossil" , "It used to eat plants" .)