A cloister is more confined without it (6)
I believe the answer is:
closer
'a cloister is more' is the definition.
I can't tell whether this definition defines the answer.
'confined without it' is the wordplay.
'confined' becomes 'close' (similar in meaning).
'without it' becomes 'r' (I can't explain this - if you can you should give a lot more credence to this answer).
'close'+'r'='CLOSER'
Can you help me to learn more?
(Other definitions for closer that I've seen before include "Not so far off" , "One winds up" , "within a shorter distance" , "Nearer in time or space" , "more precise" .)