How do I solve 12 across in Daily Telegraph Cryptic 31244?

Question

Accepted Answer

12 across reads "First half of the alphabet, something very small" (4 letters); the answer is ATOM. It's a double-definition clue: two separate definitions of the same word sit side by side. No wordplay split and no indicators — both halves of the clue define the answer. The full step-by-step breakdown is on this page.

First half of the alphabet, something very small (4)

12A — First half of the alphabet, something very small (4) — ATOM