Does something like the below work:
add_action( 'woocommerce_after_main_content', 'my_extra_description' );
function my_extra_description() {
global $wp_query;
# get the query object
$category = $wp_query->get_queried_object();
#get the cat ID
$category_ID = $category->term_id;
#get the field
$extra_description = get_field( 'extra_description', 'category_'.$category_ID );
#if we have data, show it
if( $extra_description ){
echo $extra_description;
}
}
Code untested and goes in your functions file
