Jeweler's weight (5)
I believe the answer is:
carat
'jeweler's 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 "A unit of weight for precious stones" , "Measure of fineness of gold" , "Measure of purity of gold" , "Measure for diamonds or gold" , "Weight of ice" .)