Delete watchdog messages.
drush watchdog-delete
--severity : Delete messages of a given severity level.
--type : Delete messages of a given type.
Delete all messages.
drush watchdog-delete all
Delete messages with id 64.
drush watchdog-delete 64
Delete messages containing the string "cron run succesful".
drush watchdog-delete "cron run succesful"
Delete all messages with a severity of notice.
drush watchdog-delete --severity=notice
Delete all messages of type cron.
drush watchdog-delete --type=cron