He chooses solitary confinement (7)
I believe the answer is:
recluse
'he chooses solitary confinement' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
This is the entire clue.
Can you help me to learn more?
(Other definitions for recluse that I've seen before include "company leaver" , "One preferring solitude" , "one stays away from people" , "Person living in solitude" , "One lives in solitude" .)