What does the cryptic crossword clue "Seaside clown, and maybe why his theatre is closed (7)" mean?

The wordplay uses a charade (building blocks). PIER (synonym="PIER") + ROT (synonym="ROT") = PIERROT The answer is PIERROT.

Seaside clown, and maybe why his theatre is closed (7)

Guardian Cryptic #29578 | 12 Across | 2024-12-30 flower