Wicked plate with wicked contents! (8)
I believe the answer is:
devilish
'wicked' is the definition.
(similar in meaning)
'plate with wicked contents' is the wordplay.
'plate' becomes 'dish' (type of plate).
'with' indicates putting letters inside.
'wicked contents' becomes 'evil' (I can't justify this - if you can you should give a lot more credence to this answer).
'dish' enclosing 'evil' is 'DEVILISH'.
Can you help me to learn more?
(Other definitions for devilish that I've seen before include "Diabolical" , "Mischievous; rakish" , "naughty" , "like Screwtape" , "Very difficult to deal with" .)