How do I solve 23 down in Daily Telegraph Cryptic 31253?

Question

Accepted Answer

23 down reads "Beginning of the end for POTUS" (5 letters); the answer is TRUMP. 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.

Beginning of the end for POTUS (5)

23D — Beginning of the end for POTUS (5) — TRUMP