Brilliant grade set benchmark (4,8)
I believe the answer is:
gold standard
'brilliant grade set benchmark' is the definition.
'gold standard' can be an answer for 'benchmark' (I've seen this in another clue). I am not sure about the 'brilliant grade set' bit.
'set benchmark' is the wordplay.
'set' becomes 'gold' (I can't justify this - if you can you should give a lot more credence to this answer).
'benchmark' becomes 'standard' (I've seen this before).
'gold'+'standard'='GOLD STANDARD'
Can you help me to learn more?
(Other definitions for gold standard that I've seen before include "Measure of goodness" , "Former system for defining currency value" , "Example of excellence" , "It was used to evaluate currencies" .)