16 across: Horror film keeping man in reserve before rotten finale (3, 4)
The explanation goes: keeping man in reserve before rotten finale = keeping he in tome before n = the omen = horror film.
In that, you probably noticed something questionable: reserve = tome.
No need to question it, though, it’s an error, and a very understandable one that DA himself described making in his own words:
Meanwhile 16A, the clue for THE OMEN, holds a blunder. At first draft, I’d used the word ‘book’ in the clue, which hampered surface sense. As proof phase, I thought ‘reserve’ read far better, however ‘reserve’ may signal BOOK, but never TOME. A sloppy conflation. My apologies. Feel free to file under Bullshit.
If I had my time over, my salvaged clue would read: Horror film keeping bloke in work until production’s end (3,4). But that raven has flown.