What does the cryptic crossword clue "Let walrus swim in lake (9)" mean?

The definition is "lake". The wordplay uses an anagram. anagram of LET + WALRUS = ULLSWATER; definition: "lake" The answer is ULLSWATER.

Let walrus swim in lake (9)

Guardian Cryptic #29638 | 10 Across | 2025-03-10 flower