Diamond weight measure (5)
I believe the answer is:
carat
'diamond weight measure' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for carat that I've seen before include "Measure of weight of diamonds" , "Weight of ice" , "Not a large weight" , "A unit of weight for precious stones" , "Measure of fineness of gold" .)