Green precious stone (4)
I believe the answer is:
jade
'green precious stone' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for jade that I've seen before include "Semiprecious gemstone - old, overworked horse" , "Wear down" , "weary" , "Ornamental semiprecious stone" , "Overworked horse; stone" .)