Nut’s centre (6)
I believe the answer is:
kernel
'nut's centre' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for kernel that I've seen before include "Nuts and bolts" , "Nut centre; sounds like a rank" , "Inner part of a seed or grain" , "bit of a nut" , "Essence" .)