How do I solve 20 down in Daily Telegraph Cryptic 31247?

Question

Accepted Answer

20 down reads "Sign percussion instrument can be heard" (6 letters); the answer is SYMBOL. It's a homophone clue: the answer sounds like another word. Indicators like 'reportedly', 'we hear', or 'sounds like' point to a word that's pronounced similarly to the answer. The full step-by-step breakdown is on this page.

Sign percussion instrument can be heard (6)

20D — Sign percussion instrument can be heard (6) — SYMBOL