Evil nun boxed in (8)
I believe the answer is:
sinister
'evil' is the definition.
(similar in meaning)
'nun boxed in' is the wordplay.
'nun' becomes 'sister' (a nun might be called sister).
'boxed' becomes 'ni' (I can't explain this - if you can you should give a lot more credence to this answer).
'in' means one lot of letters goes inside another.
'sister' placed around 'ni' is 'SINISTER'.
Can you help me to learn more?
(Other definitions for sinister that I've seen before include "Malign, ominous" , "Disturbing" , "Eerie, alarming" , "not right!" , "Menacing" .)