Evil nun bounds in (8)
I believe the answer is:
sinister
'evil' is the definition.
(similar in meaning)
'nun bounds in' is the wordplay.
'nun' becomes 'sister' (a nun might be called sister).
'bounds' becomes 'in' (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' going around 'in' is 'SINISTER'.
Can you help me to learn more?
(Other definitions for sinister that I've seen before include ", nun going round home" , "Forbidding" , "not right!" , "Suggesting or threatening evil or danger" , "Threatening the left in heraldry" .)