What does the cryptic crossword clue "Why one must use the stairs to unbox an item (4,3)" mean?

The definition is "Why one must use the stairs to unbox an item". The wordplay uses a double definition. Double definition The answer is LIFTOUT.

Why one must use the stairs to unbox an item (4,3)

Guardian Cryptic #29016 | 17 Down | 2023-03-13 flower