Kernel (3)
I believe the answer is:
nut
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">kernel</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for nut that I've seen before include "Fruit with hard shell" , "Hard-skinned fruit" , "eg Hazel" , "Fan" , "Screwball" .)