Underground part (4)
I believe the answer is:
root
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">underground part</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 root that I've seen before include "Basic source or origin" , "Underground growth" , "Underground part of a plant" , "top of tree? Hardly!" , "could be square" .)