Add a default value to a radio field in the WooCommerce checkout.

wc-checkout-radio-default-value.php

function custom_override_checkout_fields ( $fields ) {
	$fields['billing']['test_radio']['default'] = 'one';
	return $fields;
} // End custom_override_checkout_fields()

add_filter( 'woocommerce_checkout_fields' , 'custom_override_checkout_fields' );

I used this snippet to pre-select a radio button within the order/additional fields ($fields['order']) that I had added with the "Checkout Field Editor" plugin.

To make it work properly the filter needed to be added with an priority of at least 1001, because the "Checkout Field Editor" plugin hooks itself into with a priority of 1000.

Hope that helps outer people.