The devil's former prison? (3,4)
I believe the answer is:
old nick
'the devil's' is the definition.
(synonyms)
'former prison?' is the wordplay.
'former' becomes 'old' (both can mean previous).
'prison?' becomes 'nick' (slang term for prison).
'old'+'nick'='OLD NICK'
(Other definitions for old nick that I've seen before include "Newgate for instance" , "Scratch" , "The devil, Lucifer" , "Satan" , "Fleet, for example" .)