Diamond measure (5)
I believe the answer is:
carat
'diamond 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" , "Measure of purity of gold" , "Measure for diamonds or gold" , "Not a large weight" , "Unit of gem measurement" .)