Standard court's mark (9)
I believe the answer is:
yardstick
'standard' is the definition.
(I know that yardstick is a type of standard)
'court's mark' is the wordplay.
'court's' becomes 'yard's' (courtyard).
'mark' becomes 'tick' (I've seen this before).
'yards'+'tick'='YARDSTICK'
(Other definitions for yardstick that I've seen before include "Measure against which to make comparison" , "Measuring rod" , "Three-foot rod as standard of comparison" , "measure of performance" , "Benchmark" .)