What does the cryptic crossword clue "Why a level's rocking in rough sea (5,5)" mean?

The wordplay uses an anagram. anagram of WHY + A + LEVEL'S = HEAVYSWELL The answer is HEAVYSWELL.

Why a level's rocking in rough sea (5,5)

Daily Mail Cryptic #17217 | 11 Across | 2023-10-30 flower