The most common and most likely answer to this crossword clue is the 9 letter word STEGOSAUR
I've seen this crossword clue in The Times
S
T
E
G
O
S
A
U
R
More clues leading to the result STEGOSAUR
- Old browser I rejected as you are texting way ahead
- Lost vegetarian goats sure to stray
- Old beast I noticed that could be heard on the way
- Prehistoric lizard gets knocked back by a sour concoction
- Reptile displays pride in Tarsus recklessly
- Herbivore long ago sorted out grasses, mostly
- Beset by vast ego, Sauron was a victim of Total Wipeout
- Goats sure to rattle old reptile
- Old beast gets up and over, painful to listen to
- Old herbivore wiggling ears with gusto
Buddy explains!

The straight part
We know that "Lost vegetarian" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case lost vegetarian = stegosaur.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 STEGOSAUR mean?
STEGOSAUR noun- Any of several extinct herbivorous dinosaurs, of the suborder Stegosauria, having two rows of bony plates along the back.