How do I solve 21 across in Daily Telegraph Cryptic 31260?

Question

Accepted Answer

21 across reads "Lamps seen regularly in alcove" (4 letters); the answer is APSE. 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.

Lamps seen regularly in alcove (4)

21A — Lamps seen regularly in alcove (4) — APSE