Gemstone weight (5)
I believe the answer is:
carat
'gemstone weight' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for carat that I've seen before include "Unit of weight used for gems" , "Measure of gold in alloy" , "Unit of weight for gems and pearls" , "Measure of purity of gold" , "weight of rocks" .)