Unspeakable wickedness in beauty (8)
I believe the answer is:
devilish
'unspeakable' is the definition.
Both the answer and definition are adjectives. Perhaps they are linked in a way I don't understand?
'wickedness in beauty' is the wordplay.
'wickedness' becomes 'evil' ('evil' can be a synonym of 'wickedness').
'in' is an insertion indicator.
'beauty' becomes 'dish' (slang term for an attractive person).
'evil' placed into 'dish' is 'DEVILISH'.
Can you help me to learn more?
(Other definitions for devilish that I've seen before include "like Screwtape" , "Very difficult to deal with" , "naughty" , "Mischievous; rakish" , "Very wicked" .)