3 down: Thus turn out composer’s opening falsehoods (5)
The answer is myths. Ths is formed by removing the U of a U-turn, but the initial my? There is a composer by the name of Neil Myers, but this seems to obscure even by DA’s standards.
12, 13 across: Spooky patriarch to leave Seymour Centre, rounding shock on matriarchs (5, 6)
The answer is Gomez Addams. Go = leave, m = Seymour Centre and z = shock, but the rest?
3 Down:composer’s = my (DA is speaking self referentially)
12,13 across: rounding shock=’daze’ backwards=ezad, dams=matriarchs
I had no idea a dam was a female parent (thank you again dictionary.com).