This year (82) Rosh Chodesh Av falls on Friday and the prohibitions that apply to the Nine Days such as eating meat begin on Thursday night. If one davened Maariv by an early after Plag Minyan (and already said Yaaleh Veyavo mentioning Rosh Chodesh) can one still eat meat before the Shekiah (sunset)?

Piskei Teshuvos (551: 8) rules that it is prohibited. He also deals with the prohibition of placing clothing in a washing machine right before the Shekiah.