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