I'm still interested in knowing how to do it with an expression and will add soon files to make tests, but I finally decide to do it in another way:

• create a buffer of 1m around the buffer of the highest order (i.e 1)
• select buffers with lowest orders (i.e < 1) in the layers of buffers with all orders after v.clean
• select by location inside the previous selection buffers within the 1m buffer
• delete selected features

I'm still interested in knowing how to do it with an expression and will add soon files to make tests, but I finally decided to do it in another way:

• create a buffer of 1m around the buffer of the highest order (i.e 1)
• select buffers with lowest orders (i.e < 1) in the layers of buffers with all orders after v.clean
• select by location inside the previous selection buffers within the 1m buffer
• delete selected features