Shepherd uses goat with rare old herbivore (9)
I believe the answer is:
stegosaur
'old herbivore' is the definition.
(herbivorous dinosaur from Jurassic period)
'shepherd uses goat with rare' is the wordplay.
'shepherd' is an anagram indicator (shepherd the letters into the right order).
'with' says to put letters next to each other.
'rare' becomes 'r' (listed by Oxford Dictionary of Abbreviations).
'uses'+'goat'='usesgoat'
'usesgoat' is an anagram of 'stegosau'.
'stegosau'+'r'='STEGOSAUR'
(Other definitions for stegosaur that I've seen before include "Jurassic creature" , "prehistoric creature" , "Old veggie" , "old dinosaur" , "Armoured fossil creature" .)