Precedent setter (4,4)
I believe the answer is:
test case
'precedent setter' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for test case that I've seen before include "Court precedent" , "Legal action where the outcome will serve as a precedent" , "Precedent established" , "Forensic proof" , "that could set a precedent" .)