One is not bound to release spy (4,5)
I believe the answer is:
free agent
'one is not bound' is the definition.
I can't tell whether this defines the answer.
'release spy' is the wordplay.
'release' becomes 'free' (synonyms).
'spy' becomes 'agent' (secret agent).
'free'+'agent'='FREE AGENT'
'to' is the link.
Can you help me to learn more?
(Other definitions for free agent that I've seen before include "Someone untied by a contract" , "who would be a gift to any actor seeking work?" , "Unrestricted actor" , "Nobody can force me" , "Someone acting without constraint" .)