How do I solve 15 across in Times Cryptic 29568?

Question

Accepted Answer

15 across reads "What used to make people blue without a lot of fuss?" (4 letters); the answer is WOAD. It's a cryptic definition: the whole clue is a playful or punny description of the answer. No wordplay decomposition to do — read laterally and identify what's being described. The full step-by-step breakdown is on this page.

What used to make people blue without a lot of fuss? (4)

15A — What used to make people blue without a lot of fuss? (4) — WOAD