What are the dimensions of x and B? Presumably, A(x) has the same dimensions as B.
If x were a numerically-populated MATLAB (not CVX) variable, what MATLAB code would you use to evaluate A(x)? If you can not answer that, I doubt anyone here can help you. So far, all we know is that A(x) is a mysterious linear map from a vector to a matrix.
Is it something like A being m by n matrix, x being n by 1 vector, B being m by p matrix, and A(x) = repmat(A*x,1,p) ? If so, use minimize(norm(repmat(A*x,1,p)-B))
Whatever it is, if it is a linear map,. and you know how to evaluate it in MATLAB,. you hopefully will be able to code it the same way in CVX when x id declared a CVX variable, as opposed to being a numerically-populated MATLAB variable,.