Modern entrance for old prison (7)
I believe the answer is:
newgate
'for old prison' is the definition.
(former London prison)
'modern entrance' is the wordplay.
'modern' becomes 'new' (synonyms).
'entrance' becomes 'gate' (type of entrance).
'new'+'gate'='NEWGATE'
(Other definitions for newgate that I've seen before include "Old Nick" , "Old London prison" , "prison long ago" , "Notorious 18th-century London prison" , "London street" .)