Part of a nut (6)
I believe the answer is:
kernel
'part of a nut' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for kernel that I've seen before include "Heart and soul" , "bit of a nut" , "Nut's centre" , "Inner part of a seed or grain" , "Edible or central part of nut" .)