Underground cell (7)
I believe the answer is:
dungeon
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">underground cell</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for dungeon that I've seen before include "prison the result" , "Eg, the Black Hole of Calcutta" , "old prison" , "Prison cell, often underground" , "Unpleasant cell" .)