can i clean install os x yosemite without usb?

Yes. Restart with the Command and R keys held down, use the Disk Utility to erase the Mac OS X partition, and install Yosemite.

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Without a USB thumb drive? Yes, if you already have OS X Yosemite installed restart into the Recovery HD by pressing Command+R at start up. Then from the OS X Utilities menu select Disk Utility to erase your Macintosh HD then go back to the menu and select Reinstall OS X.

Correct.

If you don't create a USB thumb drive installer, you would upgrade to Yosemite then do what we mentioned.

I suggest you download OS X Yosemite and BEFORE you click Continue to start the upgrade install, copy the Install OS X Yosemite.app file from /Applications to another folder or external drive. OS X EL Capitan is just around the corner (Sept 30) and once that releases, Yosemite will not be available on the App Store.