How do I solve 23 across in Times Cryptic 3295?

Question

Accepted Answer

23 across reads "Dark horse’s dreadful experience" (9 letters); the answer is NIGHTMARE. It's a charade: the answer breaks into parts in clue order, each part clued by a word or short phrase. No indicators needed; watch for standard abbreviations (RA = artist, US = American, IN = at home) and short synonyms. The full step-by-step breakdown is on this page.

Dark horse’s dreadful experience (9)

23A — Dark horse’s dreadful experience (9) — NIGHTMARE