Old prison has different entrance (7)
I believe the answer is:
newgate
'old prison' is the definition.
(former London prison)
'different entrance' is the wordplay.
'different' becomes 'new' (similar 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 "Prison formerly standing on site of the Old Bailey" , "prison long ago" , "Stir in capital" , "Old Nick" , "Notorious 18th-century London prison" .)