Rigorous appraisal (4,4)
I believe the answer is:
acid test
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">rigorous appraisal</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(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" .)