We'll create fresh WordPress site with Single Post Meta Description installed. You have 20 minutes to test the plugin after that site we'll be deleted.
This plugin adds a new field in every post, where you can write a text that will be inserted in the related meta tag, located in the head section.
The main options page of the plugin, provides you with a default text area, where you can insert a default description that will appear in case the field in post page is empty.