What is it you're backing up, exchange, DC, Fileshares? And are you intending this as DR, or as a 'backup'?
There are many free ways to do this. The ghettoVCB scripts are pretty popular (though I can't vouch for them myself), the standalone vcentre converter, or just a straight shutdown and fastSCP copy to different storage.
It really all depends on your budget, or lack of, and whether you need to do this live or if down time isn't an issue.
If cloning is absolutely what you must do, then you need to buy vCentre and install it (either on a windows server or using the available linux appliance). You cannot clone a machine when directly connected to an ESXi host.
I've checked on our work servers and there is no option to do it, despite them being fully licensed.
There are a ton of ways to achieve what you asked, but in order to advise you better we'd need to know more fully what it is your perceived end goal is, as cloning may not be the right answer.