Subterranean (11)
I believe the answer is:
underground
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">subterranean</span>' is the definition.<br/ ><span class="explanation-format__explanation">(synonyms)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for underground that I've seen before include "Travel on this" , "Subterranean" , "Secret resistance movement" , "Tube; covert" , "Transport system" .)