Underground vault (5)
I believe the answer is:
crypt
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">underground vault</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 crypt that I've seen before include "Underground chapel" , "Cellar in church" , "Underground burial chamber" , "Tomb" , "Underground room of church" .)