How to limit the amount of entries that is output when limit doesn't work
What you probably want is to order them by date and than select the first.

{% set news = craft.entries.section('news').order('postDate desc').first() %}


And if you want to use limit somewhere (but I don't think this is what you are trying to get here) you should add limit(1) to this {% set news = craft.entries.section('news').limit(1) %}