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It probably depends upon what the ingredients are in the tart, and how it has been stored. When you received it, was it frozen? If it was frozen and you've thawed it out, you probably can heat the part you want and refreeze the rest. It wil likely suffer some in quality. I wouldn't leave it in the freezer more than a week.
Has the tart already been baked? or is it one you are supposed to bake? If you bake it off then you can cut a section to freeze, no problem.
OTOH, if it was already baked and given to you, I'd invite a few folks over to help eat it and not try to save it for another time.