The after_setup_theme hook is actually an action hook not a filter hook. Nonetheless that should not be needed.
Give this a try:
if( ! function_exists( 'get_field' ) ) {
function get_field( $key, $post_id = false, $format_value = true ) {
if( false === $post_id ) {
global $post;
$post_id = $post->ID;
}
return get_post_meta( $post_id, $key, true );
}
}