WooCommerce - customise a product tab

functions.php

add_filter( 'woocommerce_product_tabs', 'woo_custom_description_tab', 98 );
function woo_custom_description_tab( $tabs ) {

	$tabs['description']['callback'] = 'woo_custom_description_tab_content';	// Custom description callback

	return $tabs;
}

function woo_custom_description_tab_content() {
	echo '<h2>Custom Description</h2>';
	echo '<p>Here\'s a custom description</p>';
}