Object in white, moonstone (4)
I believe the answer is:
item
'object' is the definition.
(both can mean a thing)
'in white moonstone' is the wordplay.
'in' says the answer is hidden in the clue.
'ITEM' is hidden in the letters of 'white moonstone'.
(Other definitions for item that I've seen before include "date" , "One thing on a list" , "Seperate article in an enumeration" , "Book-keeping entry" , "One unit on a list" .)