Rigorous appraisal (4,4)
I believe the answer is:
acid test
'rigorous appraisal' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for acid test that I've seen before include "Dictates to undergo conclusive trial" , "Conclusive moment (using litmus paper?)" , "Conclusive trial of success or value" , "Ultimate proof" , "crucial experiment" .)