Unit of weight for precious stones (5)
I believe the answer is:
carat
'unit of weight for precious stones' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for carat that I've seen before include "Measure of weight of diamonds" , "Weight of ice" , "Precious stone weight" , "Not a large weight" , "Measure of purity of gold" .)