Underground (12)
I believe the answer is:
subterranean
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">underground</span>' is the definition.<br/ ><span class="explanation-format__explanation">(synonyms)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for subterranean that I've seen before include "Underground; secret" , "beneath the waves" .)