Article in white marble (4)
I believe the answer is:
item
'article' is the definition.
(both can mean an object or thing)
'in white marble' is the wordplay.
'in' indicates a hidden word.
'ITEM' is hidden in the letters of 'white marble'.
(Other definitions for item that I've seen before include "Time (anag.)" , "One article" , "Two lovers as one" , "One thing on a list" , "Also no more" .)