Spot erratic, wild dinosaur (11)
I believe the answer is:
triceratops
'dinosaur' is the definition.
(triceratops is a kind of dinosaur)
'spot erratic wild' is the wordplay.
'wild' indicates an anagram.
'spot'+'erratic'='spoterratic'
'spoterratic' with letters rearranged gives 'TRICERATOPS'.
(Other definitions for triceratops that I've seen before include "Extinct reptile" , "Three-horned dinosaur" , "Old creature" , "Horned dinosaur" , "Cretaceous herbivore" .)