How to slice a struct array?

No problem - just use :

arr = [x.a];

It will concat all of the values that you need. If you have a more complex data, you can use the curly bracers:

b(1).x = 'John';
b(2).x = 'Doe';
arr = {b.x}; 

For a multi-dimensional array, you need

reshape([x.a], size(x))