Waste one new cell (7)
I believe the answer is:
dungeon
'cell' is the definition.
(underground cell)
'waste one new' is the wordplay.
'waste' becomes 'dung' (dung is a kind of waste).
'new' indicates an anagram.
'one' with letters rearranged gives 'eon'.
'dung'+'eon'='DUNGEON'
(Other definitions for dungeon that I've seen before include "Dark underground cell for prisoners" , "Underground prison-cell" , "old prison" , "part of prison" , "Dark place of confinement" .)