Waste long period of time in US prison (7)
I believe the answer is:
dungeon
'prison' is the definition.
(a dungeon is a prison cell)
'waste long period of time in us' is the wordplay.
'waste' becomes 'dung' (dung is a kind of waste).
'long period of time in us' becomes 'eon' (US spelling of 'aeon').
'dung'+'eon'='DUNGEON'
(Other definitions for dungeon that I've seen before include "Underground prison-cell" , "Dark underground cell for prisoners" , "Uncongenial accommodation" , "Unpleasant cell" , "prison the result" .)