So you might want to use a WordPress post's excerpt as the meta description for pages featuring a single post.
Using the_excerpt() wraps the content in <p></p> tags - which is rubbish, and a little bewildering to be honest. Don't bother using the hacky methods of stripping off a set number of characters from each end in PHP; that's just ridiculous.
Use the_excerpt_rss() and you'll get the same returned as a plain text string. No HTML tags included. Lovely.