Truthfully, this one could go in General Questions. There is actually a factually correct answer to the OP.
For sex with a clone to constitute “masturbation”, your clone would have to be considered part of you. There is no basis for such an assumption. A clone is a separate person from you, genetically identical, but with different experiences, memories, thoughts, and a different (though likely similar) personality. A clone would be in some ways like a twin sibling, but a clone would be younger than you by however many years old you were at the time the clone was made–a clone of an adult would not, of course, be a full-sized identical copy of that adult; a clone would be a zygote which would develop into a baby. A clone formed from an adult could be seen, morally speaking, as more like a child of that adult than a sibling. If two clones were formed from an adult at the same time, they would be siblings of each other. In either case, clone sex would be a form of incest–a cross between sibling incest and parent-child incest in the first case; and incest between twin siblings in the second case. (Of course, barring some genetic manipulations even more complex than simple cloning, a clone will be of the same sex as the parent organism, so we’re not talking about the sort of incest which could result in inbred children.) If an adult had himself or herself cloned, sex with the clone might also constitute child molestation, if the clonee didn’t wait long enough before initiating the relationship.