Waste one plan for prison (7)
I believe the answer is:
dungeon
'prison' is the definition.
(thesaurus)
'waste one plan' is the wordplay.
'waste' becomes 'dung' (dung is a kind of waste**).
'plan' is an anagram indicator (plan can mean organise).
'one' anagrammed gives 'eon'.
'dung'+'eon'='DUNGEON'
'for' is the link.
(Other definitions for dungeon that I've seen before include "Uncongenial accommodation" , "Unpleasant cell" , "Eg, the Black Hole of Calcutta" , "Dark place of confinement" , "part of prison" .)