Popular pie nut
I believe the answer is:
pecan
'popular pie nut' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for pecan that I've seen before include "Species of hickory - edible nut" , "Something like a walnut" , "Edible nut like a smooth walnut" , "US nut tree" , "Smooth pinkish-brown nut, nice in pie" .)