Nuclei (5)

I believe the answer is:
cores
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">nuclei</span>' is the definition.<br/ ><span class="explanation-format__explanation">('core' can be a synonym of 'nucleus')</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for cores that I've seen before include "Central parts" , "Hearts" , "The centres of objects" .)