Conclusive examination (4,4)
I believe the answer is:
acid test
'conclusive examination' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for acid test that I've seen before include "Validity confirmed" , "Conclusive moment (using litmus paper?)" , "Rigorous or crucial appraisal" , "crucial trial" , "Conclusive proof" .)