Solution: The six themer entries (clues that begin “Start for”) could also be the answers for six of the down clues- 15A could be the answer for 8D, 23A & 29D, 38A & 39D, 40A & 12D, 50A & 52D, 63A & 10D. The first letters of the corresponding down entries spell BEFORE, the meta answer.
Another super meta, LarsCaine! I may have mentioned this before (feeble memory ), but we're always looking for new constructors on the X-Word Muggles forum. Please email me at rakennedy16 @ gmail.com if you're ever interested. Thanks!