Old prison has modern entrance (7)
I believe the answer is:
newgate
'old prison' is the definition.
(former London prison)
'modern entrance' is the wordplay.
'modern' becomes 'new' (associated in meaning).
'entrance' becomes 'gate' (type of entrance).
'new'+'gate'='NEWGATE'
'has' acts as a link.
(Other definitions for newgate that I've seen before include "Old Nick" , "Prison formerly standing on site of the Old Bailey" , "London street" , "Old London prison" , "Stir in capital" .)