One crater formed, and no more dinosaurs (11)
I believe the answer is:
triceratops
'one crater formed and no more dinosaurs' is the definition.
I can't judge whether this defines the answer.
'one crater formed and no more dinosaurs' is the wordplay.
'one' becomes 'i' (Roman numeral).
'formed' indicates anagramming the letters.
'and' means one lot of letters go next to another.
'no more dinosaurs' becomes 'tops' (I can't justify this - if you can you should give a lot more credence to this answer).
'i'+'crater'='icrater'
'icrater' anagrammed gives 'tricera'.
'tricera'+'tops'='TRICERATOPS'
Can you help me to learn more?
(Other definitions for triceratops that I've seen before include "Horned dinosaur" , "Extinct reptile" , "Cretaceous herbivore" , "Old creature" , "Three-horned dinosaur" .)