How do I solve 6 across in Daily Telegraph Cryptic 3370?

Question

Accepted Answer

6 across reads "Opening letters" (4 letters); the answer is POST. 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.

Opening letters (4)

6A — Opening letters (4) — POST