How do I solve 3 down in Daily Telegraph Cryptic 31244?

Question

Accepted Answer

3 down reads "Kiln starts to overheat after setting temperature" (4 letters); the answer is OAST. 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.

Kiln starts to overheat after setting temperature (4)

3D — Kiln starts to overheat after setting temperature (4) — OAST