How do I solve 16 down in Daily Telegraph Cryptic 31270?

Question

Accepted Answer

16 down reads "Prop in bar with lock" (8 letters); the answer is BUTTRESS. 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.

Prop in bar with lock (8)

16D — Prop in bar with lock (8) — BUTTRESS