Avant-garde musical artist Yoko

ONO

This Japanese multimedia artist and singer was a key figure in the Fluxus movement and famously collaborated with John Lennon. Her last name is also a homophone for a number in Spanish.

This clue points to Yoko Ono, the famous avant-garde artist and musician known for her experimental work and marriage to John Lennon. "Avant-garde" hints at her boundary-pushing style, while "musical artist" narrows it down to her rather than other figures named Yoko. The answer is simply her last name, "ONO," which fits neatly into crossword grids.