You should start immediately and gather your information on this transaction. If you shipped to a PAYPAL confirmed address and you can provide an online tracking showing delivery to the paypal confirmed address then you are covered under the PAYPAL SELLER PROTECTION POLICY. However keep in mind that if the sale value is over US$250 then you also have to have a SIGNATURE.
If you shipped the goods without online tracking proof then you are S.O.L.
A bank clawback is usually instigated by a buyer when they know that the seller is vulnerable because they shipped the item using a cheap delivery method. Also, it could be that the true owner of the bank account used to pay for the item has had their account hijacked by a friend and/or relative.
Eitherway call PAYPAL and get more information as to why this is occuring and what they need/expect from you.
Good Luck!
TIM @ FTL