Waste many years in prison (7)
I believe the answer is:
dungeon
'prison' is the definition.
(a dungeon is a prison cell)
'waste many years' is the wordplay.
'waste' becomes 'dung' (dung is a kind of waste).
'many years' becomes 'eon' (I've seen this in another clue).
'dung'+'eon'='DUNGEON'
'in' acts as a link.
(Other definitions for dungeon that I've seen before include "Unpleasant cell" , "part of prison" , "Eg, the Black Hole of Calcutta" , "Underground prison-cell" , "Dark prison cell" .)