How do I solve 1 down in Daily Telegraph Cryptic 31241?

Question

Accepted Answer

1 down reads "First Lady's partner's an old president" (5 letters); the answer is ADAMS. 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.

First Lady's partner's an old president (5)

1D — First Lady's partner's an old president (5) — ADAMS