Waste time in prison (7)
I believe the answer is:
dungeon
'prison' is the definition.
(thesaurus)
'waste time' is the wordplay.
'waste' becomes 'dung' (dung is a kind of waste).
'time' becomes 'eon' (eon is a kind of time**).
'dung'+'eon'='DUNGEON'
'in' acts as a link.
(Other definitions for dungeon that I've seen before include "Prison cell, often underground" , "Place of confinement underground" , "Keep" , "Dark prison cell" , "prison the result" .)