The most common and most likely answer to this crossword clue is the 9 letter word IGUANODON
I've seen this crossword clue in The Guardian
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More clues leading to the result IGUANODON
- Old animal droppings gathered by a single lecturer
- Ancient creature’s droppings covered by one lecturer
- In the past a massive herbivore, one dropping kebab without hesitation
- Dinosaur dung collected by institute’s top academic
- Waste of animal in dinosaur not half tragic, reptile now extinct
- Poop deck only starts flooding in for old monster
- Dinosaur poo collected by international academic
- I poop on teacher, old fossil?
- I waste positive gesture backing old reptile
- In eating droppings, function as dinosaur
Buddy explains!

The straight part
We know that "old fossil" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case old fossil = iguanodon.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 IGUANODON mean?
IGUANODON noun- Any of several large dinosaurs, of the genus Iguanodon, of the Jurassic and Cretaceous periods