How do I solve 25 down in Times Cryptic 29556?

Question

Accepted Answer

25 down reads "Two blood types primarily form clot" (3 letters); the answer is OAF. 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.

Two blood types primarily form clot (3)

25D — Two blood types primarily form clot (3) — OAF