### Clue: It really discriminates against a trial involving detectives (4,4)
### Answer: ACID TEST 
 ### Explanation: Definition: It really discriminates
  A TEST(a trial) **containing** CID(detectives from the $$C$$riminal $$I$$nvestigation $$D$$epartment, Scotland Yard)
Answer:  A rigorous and conclusive test,really discriminates to establish worth or value, derived from the old method of testing gold with concentrated nitric acid