The inner peel of a citrus fruit (4)
I believe the answer is:
pith
(Other definitions for pith that I've seen before include "White inner skin of an orange" , "Tissue beneath citrus fruit rind" , "Tissue inside the rind of citrus fruits" , "Lining of rind; essence" , "Spongy tissue lining rinds of oranges, or the essential part" .)