What does the cryptic crossword clue "Whiskery aquatic mammal in the deep lake (4)" mean?

The definition is "lake". The wordplay uses a charade (building blocks). SEA (synonym="”the deep”") + L (synonym="lake") = SEAL The answer is SEAL.

Whiskery aquatic mammal in the deep lake (4)

Guardian Cryptic #29489 | 21 Down | 2024-09-16 flower