Underground explorer (5)
I believe the answer is:
caver
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">underground explorer</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for caver that I've seen before include "Potholer" , "Speleologist" , "Underground explorer" .)