What does the cryptic crossword clue "What's causing drop in level of eg a lake somehow? (7)" mean?

The wordplay uses an anagram. anagram of EG + A + LAKE = LEAKAGE The answer is LEAKAGE.

What's causing drop in level of eg a lake somehow? (7)

Daily Mail Cryptic #16790 | 2 Down | 2022-03-10 flower