The most common and most likely answer to this crossword clue is the 6 letter word COFFIN
I've seen this crossword clue in Financial Times
C
O
F
F
I
N
Buddy explains!

The straight part
We know that "box" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case box = coffin.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 COFFIN mean?
COFFIN noun- A rectangular closed box in which the body of a dead person is placed for burial.
- The eighth Lenormand card.
- A basket.
- A casing or crust, or a mold, of pastry, as for a pie.
- A conical paper bag, used by grocers.
- The hollow crust or hoof of a horse's foot, below the coronet, in which is the coffin bone.
- A storage container for nuclear waste.
- To place in a coffin.