Barrier in former London prison, not new (4)
I believe the answer is:
gate
'barrier' is the definition.
(gate is a kind of barrier)
'former london prison not new' is the wordplay.
'former london prison' becomes 'newgate' (I've seen this before).
'not' is a deletion indicator.
'newgate' with 'new' taken out is 'GATE'.
'in' is the link.
(Other definitions for gate that I've seen before include "Portal" , "Number of people admitted to a sporting event" , "Movable barrier in a fence or wall" , "Airport boarding point" , "Garden door?" .)