Agent we tortured in prison once (7)
I believe the answer is:
newgate
'prison once' is the definition.
(historical prison)
'agent we tortured' is the wordplay.
'tortured' indicates an anagram.
'agent'+'we'='agentwe'
'agentwe' with letters rearranged gives 'NEWGATE'.
'in' is the link.
(Other definitions for newgate that I've seen before include "Prison formerly standing on site of the Old Bailey" , "Old London prison" , "Old Nick" , "prison long ago" , "Former London prison" .)