The most common and most likely answer to this crossword clue is the 11 letter word TYRANNOSAUR
I've seen this crossword clue in The Times
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Buddy explains!

The straight part
We know that "an old monster" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case an old monster = tyrannosaur.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.
Crossword Answer definitions
TYRANNOSAUR noun- Any large bipedal carnivorous dinosaur, of the family Tyrannosauridae, that lived in North America during the Cretaceous period.