The most common and most likely answer to this crossword clue is the 13 letter word TRICERATOPSES
I've seen this crossword clue in The Daily Telegraph
T
R
I
C
E
R
A
T
O
P
S
E
S
More clues leading to the result TRICERATOPSES
Buddy explains!

The straight part
We know that "Dinosaurs" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case dinosaurs = triceratopses.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does TRICERATOPSES mean?
TRICERATOPSES noun- Common name of the extinct genus Triceratops; a herbivorous ceratopsid from the late Cretaceous.