Soft tissue in bones (6)
I believe the answer is:
marrow
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">soft tissue in bones</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for marrow that I've seen before include "Edible gourd" , "Stuff found inside bones" , "essential part" , "Fatty stuff in bones or large fruit" , "Large green squash" .)