What does the cryptic crossword clue "Horribly wet — a valid reason? (5,4)" mean?

The definition is "wet / a valid reason". The wordplay uses an anagram. anagram of WET A VALID = TIDAL WAVE The answer is TIDAL WAVE.

Horribly wet — a valid reason? (5,4)

Times Cryptic #28647 | 25 Across | 2023-07-05 flower