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
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