Nut’s centre (6)
I believe the answer is:
kernel
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">nut's centre</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(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" .)