How do I solve 22 across in Daily Telegraph Cryptic 31239?

Question

Accepted Answer

22 across reads "Shrub expert's seen by river" (4 letters); the answer is ACER. 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.

Shrub expert's seen by river (4)

22A — Shrub expert's seen by river (4) — ACER