Treat a critter with surgical procedures, old herbivore (11)
I believe the answer is:
triceratops
'old herbivore' is the definition.
(herbivore dinosaur)
'treat a critter with surgical procedures' is the wordplay.
'treat' is an anagram indicator.
'with' means one lot of letters go next to another.
'surgical procedures' becomes 'ops' (op is short for operation).
'a'+'critter'='acritter'
'acritter' is an anagram of 'tricerat'.
'tricerat'+'ops'='TRICERATOPS'
(Other definitions for triceratops that I've seen before include "Cretaceous herbivore" , "Extinct reptile" , "Old creature" , "Three-horned dinosaur" , "Horned dinosaur" .)