If you are the admin of Power Portals and you want to clear cache, Follow the  below steps :

Step-1:Login power portal by  using Azure AD login option and provide global administrator credential and login.

Step-2:Open another Tab in your  browser and use your portal URL and append /_services/about and click  enter.

For Eg:https://grm.powerappsportals.com/_services/about

Step-3: Then the portal details page will be open. Click on the Clear Cache button to clear the cache.

Now after some seconds the cache will be cleared.