term for i believe ill have just another crumb of that lemon cream pie
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The term you are referring to is "one more bite" or "one more slice" of lemon cream pie. This phrase implies that you want to have a little bit more of the pie, even though you might have had enough.
If you are looking for a specific term, it is worth noting that there might not be a widely recognized phrase or expression specifically referring to having just a small additional portion of a lemon cream pie. However, you could simply say "May I have a small extra serving of the lemon cream pie, please?" or "I'd like to have just a tiny bit more of the lemon cream pie."