How do I solve 6 down in Express Cryptic 20260519?

Question

Accepted Answer

6 down reads "Chief less sensitive to three, say" (5,6 letters); the answer is PRIME NUMBER. It's a charade: the answer breaks into parts in clue order, each part clued by a word or short phrase. No indicators needed; watch for standard abbreviations (RA = artist, US = American, IN = at home) and short synonyms. The full step-by-step breakdown is on this page.

Chief less sensitive to three, say (5,6)

6D — Chief less sensitive to three, say (5,6) — PRIME NUMBER