Pith (6)
I believe the answer is:
marrow
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">pith</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I know that pith can be written as marrow)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for marrow that I've seen before include "Large courgette" , "Vegetable; bone filling" , "Large green squash" , "The best part" , "Soft bone tissue" .)