Novel barrier to one-time notorious prison (7)
I believe the answer is:
newgate
'one-time notorious prison' is the definition.
'Newgate' can be an answer for 'prison' (I've seen this before). I am not sure about the 'one-time notorious' bit.
'novel barrier' is the wordplay.
'novel' becomes 'new' (synonyms).
'barrier' becomes 'gate' (gate is a kind of barrier).
'new'+'gate'='NEWGATE'
'to' is the link.
Can you help me to learn more?
(Other definitions for newgate that I've seen before include "Notorious 18th-century London prison" , "prison long ago" , "Former London prison" , "Old London prison" , "Prison formerly standing on site of the Old Bailey" .)