How do I solve 21 down in Daily Telegraph Cryptic 31242?

Question

Accepted Answer

21 down reads "Loud noise from family dog's bottom" (5 letters); the answer is CLANG. It's a letter-selection clue: take specific letters from a longer source. 'First of' means the opening letter, 'extremely' means first + last, 'regularly' means every other letter, 'oddly' / 'evenly' means odd or even positions. The full step-by-step breakdown is on this page.

Loud noise from family dog's bottom (5)

21D — Loud noise from family dog's bottom (5) — CLANG