Clean ESN simply means that the phone is not currently tied to a phone number. Basically if you bought the phone at Verizon, activated it, and then went back to using your old phone the ESN is clean.
To ensure that it is clean, you can call verizon I believe and have them check. If you ordered the phone online, you need to activate on the phone line you ordered it for and then deactivate it by activating another phone on that number.