Production Assistants are advised to wear comfortable, utilitarian clothing that makes sense for hours on set, standing on your feet, and running around as a gofer.
Most Set PAs wear jeans and a t-shirt or sweater, possibly with a hat and sunscreen if they’re working outside. Office PAs dress business casual, with a nice shirt and jeans, slacks, or a skirt.
Dress for movement, in a way that inspires confidence!