I want to hide category description when some filter is applied.
You can hide category description if any filter is applied in the file
app\design\frontend\<your-package>\<your-theme>\template\catalog\category\view.phtml
just add a condition
<?php if (!Mage::helper('core')->isModuleEnabled('Amasty_Shopby') || !Mage::helper('amshopby/attributes')->getRequestedFilterCodes()):?>
...
... category title and description code ...
...
<?php endif ?>
See more details on Improved Layered Navigation page
