Sweet potato (3)
(Other definitions for yam that I've seen before include "May be a sweet spud" , "vegetable plant" , "Potato-like vegetable" , "May be a sweet potato" , "Tropical root vegetable" .)
(Other definitions for pie that I've seen before include "A baked dish covered with pastry" , "Type of chart" , "Pastry crust with filling" , "something with custard, say" , "Pastry product" .)