Waste material one’s thrown in underground cell (7)
I believe the answer is:
dungeon
'underground cell' is the definition.
(I've seen this before)
'waste material one's thrown' is the wordplay.
'waste material' becomes 'dung' (dung is a kind of waste material).
'thrown' is an anagram indicator.
'one' anagrammed gives 'eon'.
'dung'+'eon'='DUNGEON'
'in' acts as a link.
(Other definitions for dungeon that I've seen before include "Underground prison-cell" , "part of prison" , "Prison cell, often underground" , "Dark underground cell for prisoners" , "Dark place of confinement" .)