how to clear phone from virus? is this any free feature for remove virus? tell me how to clear cache?i am having in trouble

You got tricked into installing a rogue calendar; your phone doesn’t have a virus. It has an annoying ad whose purpose is to get you to either log into a site that will steal your data or to download a useless, expensive app that may also steal your data.

If your phone has iOS 13 or earlier: Go to Settings/Passwords & Accounts, look under Subscribed Calendars, tap on the offending calendar and delete it.

If your phone has iOS 14: Go to Settings/Calendar/Accounts, Tap Subscribed Calendars, tap on the offending calendar and delete it.

And in the future beware of "free" offers

Settings > Calendar > Accounts

Any rogue entries in there, delete them.

The cache cannot be cleared on an iPhone to my knowledge.