Milky gemstone (4)
I believe the answer is:
opal
'milky gemstone' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for opal that I've seen before include "Delicately coloured gem" , "Iridescent quartz-like stone" , "Semi-transparent gem" , "Milky-white gemstone with flashes of colour" , "Type of gem" .)