How do I solve 18 down in Daily Telegraph Cryptic 31264?

Question

Accepted Answer

18 down reads "Fence becoming weaker" (6 letters); the answer is PALING. 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.

Fence becoming weaker (6)

18D — Fence becoming weaker (6) — PALING