Rhizome (9)
I believe the answer is:
rootstock
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">rhizome</span>' is the definition.<br/ ><span class="explanation-format__explanation">(synonyms)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for rootstock that I've seen before include "Source" , "Rhizome" , "that extends underground" .)