The most common and most likely answer to this crossword clue is the 5 letter word HILLS
H
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L
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More clues leading to the result HILLS
Buddy explains!

The straight part
We know that "they are/proverbially old" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case they are/proverbially old = hills.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 HILLS mean?
HILLS noun- An elevated location smaller than a mountain.
- A sloping road.
- A heap of earth surrounding a plant.
- A single cluster or group of plants growing close together, and having the earth heaped up about them.
- The pitcher’s mound.
- To form into a heap or mound.
- To heap or draw earth around plants.