The most common and most likely answer to this crossword clue is the 12 letter word ANTEDILUVIAN
I've seen this crossword clue in The Times
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More clues leading to the result ANTEDILUVIAN
Buddy explains!

The straight part
We know that "Very old" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case very old = antediluvian.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 ANTEDILUVIAN mean?
ANTEDILUVIAN noun- One who lived prior to Noah's Flood.
- Ancient or antiquated.
- Extremely dated.
- Pertaining or belonging to the time period prior to a great or destructive flood or deluge.
- Pertaining or belonging to the time prior to Noah's Flood.