Underground prison (7)
I believe the answer is:
dungeon
'underground prison' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for dungeon that I've seen before include "Eg, the Black Hole of Calcutta" , "Unpleasant cell" , "Prison cell, often underground" , "Dark underground cell for prisoners" , "Place of confinement underground" .)