What does the cryptic crossword clue "U.S. vegetable? Swap one if rotten (4,3)" mean?

The wordplay uses an anagram. anagram of SWAP + ONE = SNOWPEA The answer is SNOWPEA.

U.S. vegetable? Swap one if rotten (4,3)

Daily Mail Cryptic #17220 | 27 Across | 2023-11-02 flower