The most common and most likely answer to this crossword clue is the 10 letter word SARCOPHAGI
I've seen this crossword clue in The Guardian
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Buddy explains!

The straight part
We know that "old boxes" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case old boxes = sarcophagi.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
SARCOPHAGI noun- A stone coffin, often inscribed or decorated with sculpture.
- The cement and steel structure that encases the destroyed reactor at the power station in Chernobyl, Ukraine.
- A kind of limestone used by the Greeks for coffins, so called because it was thought to consume the flesh of corpses.
- An 18th-century form of wine cooler.