Mammalian martial arts instructor of the Teenage Mutant Ninja Turtles (8)
I believe the answer is:
splinter
(Other definitions for splinter that I've seen before include "Sharp piece of glass or metal - shatter" , "Thin needle-like piece" , "Shiver; break" , "Thin sharp piece of wood or glass" , "Sliver of wood, sore in the flesh" .)