One given discretion to let spy out of prison (4,5)
I believe the answer is:
free agent
'one given discretion' is the definition.
'out of prison' is the wordplay.
'out' becomes 'free' (I've seen this before).
'of prison' becomes 'agent' (I can't justify this - if you can you should believe this answer much more).
'free'+'agent'='FREE AGENT'
'to let spy' is the link.
I am not very happy about this link. Some or all of it may belong to another bit of the clue.
(Other definitions for free agent that I've seen before include "Someone whose actions are not constrained by others" , "Nobody can force me" , "One acting without constraint" , "who would be a gift to any actor seeking work?" , "One unrestrained" .)