Old prison's modern entrance (7)
I believe the answer is:
newgate
'old prison's' is the definition.
(former London prison)
'modern entrance' is the wordplay.
'modern' becomes 'new' ('new' is associated in meaning with 'modern').
'entrance' becomes 'gate' (I've seen this before**).
'new'+'gate'='NEWGATE'
(Other definitions for newgate that I've seen before include "Former London prison" , "London street" , "Old London prison" , "Stir in capital" , "prison long ago" .)