How do I solve 18 down in Daily Telegraph Cryptic 31263?

Question

Accepted Answer

18 down reads "US artist having a peep?" (8 letters); the answer is WHISTLER. It's a double-definition clue: two separate definitions of the same word sit side by side. No wordplay split and no indicators — both halves of the clue define the answer. The full step-by-step breakdown is on this page.

US artist having a peep? (8)

18D — US artist having a peep? (8) — WHISTLER