add post content in meta description in yoast

Here is a safe and Yoast preferred method

add_action('wp_head','add_custom_meta_description_box');

function retrieve_var1_replacement( $var1 ) {
        global $post;
       return strip_tags($post->post_content);
}
function register_my_plugin_extra_replacements() {
       wpseo_register_var_replacement( '%%mycustomdesc%%', 'retrieve_var1_replacement', 'advanced', 'this is a help text for myvar1' );
}
add_action( 'wpseo_register_extra_replacements', 'register_my_plugin_extra_replacements' );

You can now replace your %%excerpt%% with %%mycustomdesc%%