A measure of the gold standard (5)
I believe the answer is:
carat
This might be a double definition.
'a measure' is the first definition.
(carat is a kind of measure)
'gold standard' is the second definition.
The definition and answer can be both quantities as well as being singular nouns.
Perhaps you can see an association between them that I can't see?
'of the' acts as a link.
Can you help me to learn more?
(Other definitions for carat that I've seen before include "Unit of weight of gemstones" , "Precious stone weight" , "Unit of weight for gems or purity of gold" , "Unit of gem measurement" , "Unit of weight for gems and pearls" .)