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?