Drush won't clear uploaded temporary files
Simply make CRON run drush as _www user. /etc/cron.d suppotys it, it's format is:

#&lt;timing&gt;   &lt;user&gt; &lt;command&gt;


Just make entry for your drush:

11 * * * *    _www   drush


Details, of course, may vary a little.

I used this answer.