1) Extras in The Last Samurai 2) Fictional teenage turtles 3) Japanese assassins of old 4) Japanese mercenaries 5) Japanese warriors 6) Martial arts experts 7) Martial arts masters 8) Masked fighters 9) Mercenaries of yore 10) Oriental warriors 11) Silent assassins 12) Sneakers from long ago 13) Stealthy fighters Found on https://www.crosswordclues.com/clue/ninjas