Herbivorous dinosaur (9)
'herbivorous dinosaur' is the definition.
(herbivorous dinosaur from Jurassic period)
This is the entire clue.
(Other definitions for stegosaur that I've seen before include "Armoured fossil creature" , "Old beast" , "old dinosaur" , "Old veggie" , "Jurassic creature" .)
(Other definitions for triceratops that I've seen before include "Horned dinosaur" , "Cretaceous herbivore" , "Old creature" , "Three-horned dinosaur" , "Extinct reptile" .)