Novel entrance to old prison (7)
I believe the answer is:
newgate
'to old prison' is the definition.
(former London prison)
'novel entrance' is the wordplay.
'novel' becomes 'new' (novel means new or fresh).
'entrance' becomes 'gate' (type of entrance).
'new'+'gate'='NEWGATE'
(Other definitions for newgate that I've seen before include "prison long ago" , "Prison formerly standing on site of the Old Bailey" , "Notorious 18th-century London prison" , "Former London prison" , "Stir in capital" .)