Instant trap so designed for dinosaur (11)
I believe the answer is:
triceratops
'dinosaur' is the definition.
(triceratops is a kind of dinosaur)
'instant trap so designed' is the wordplay.
'instant' becomes 'trice' (synonyms).
'designed' indicates an anagram (designed with the letters).
'trap'+'so'='trapso'
'trapso' with letters rearranged gives 'ratops'.
'trice'+'ratops'='TRICERATOPS'
'for' is the link.
(Other definitions for triceratops that I've seen before include "Three-horned dinosaur" , "Cretaceous herbivore" , "Extinct reptile" , "Horned dinosaur" , "Old creature" .)