Remove a div from RSS feed

You can use the_content_feed hook for that. It is used to filter the content of the post after it is retrieved from the database and filtered by “the_content” and before it is sent to RSS reader (or browser).

So here’s the code:

add_filter( 'the_content_feed', function ( $content ) {
    $content = str_replace( '<div class="well"> <a class="btn" href="https://wordpress.stackexchange.com/questions/330141/foo.com">foo</a></div>', '', $content );
    return $content;
} );