Wicked dresses are slightly reduced, second-hand (9)
I believe the answer is:
vicarious
'second-hand' is the definition.
(I've seen this before)
'wicked dresses are slightly reduced' is the wordplay.
'wicked' becomes 'vicious' (similar in meaning).
'dresses' means one lot of letters goes inside another (dress can mean cover).
'are slightly reduced' becomes 'ar' (I can't explain this - if you can you should believe this answer much more).
'vicious' enclosing 'ar' is 'VICARIOUS'.
Can you help me to learn more?
(Other definitions for vicarious that I've seen before include "taking another's place" , "Acting for others" , "Experienced at secondhand" , "Acting for another" , "Indirect" .)