Waste a long time in underground prison (7)
I believe the answer is:
dungeon
'underground prison' is the definition.
(I've seen this before)
'waste a long time' is the wordplay.
'waste' becomes 'dung' (dung is a kind of waste).
'a long time' becomes 'eon' (eon is a kind of long time).
'dung'+'eon'='DUNGEON'
'in' acts as a link.
(Other definitions for dungeon that I've seen before include "Eg, the Black Hole of Calcutta" , "Dark prison cell" , "Dark place of confinement" , "Uncongenial accommodation" , "Prison cell, often underground" .)