How do I solve 20 across in Daily Telegraph Cryptic 3374?

Question

Accepted Answer

20 across reads "Device attached to a killer gag?" (8 letters); the answer is SILENCER. 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.

Device attached to a killer gag? (8)

20A — Device attached to a killer gag? (8) — SILENCER