What does the cryptic crossword clue "A better law altered what hydrologists study (5,5)" mean?

The wordplay uses an anagram. anagram of A + BETTER + LAW = WATERTABLE The answer is WATERTABLE.

A better law altered what hydrologists study (5,5)

Daily Mail Cryptic #16734 | 23 Across | 2021-12-22 flower