Providing fallback function and allow override by plugin

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 );
    }
}