Police in corrupt state giving conclusive trial (4,4)
I believe the answer is:
acid test
'conclusive trial' is the definition.
(I've seen this before)
'police in corrupt state' is the wordplay.
'police' becomes 'CID' (abbreviation for Criminal Investigation Department).
'in' indicates putting letters inside.
'corrupt' indicates an anagram (a corrupted form).
'state' anagrammed gives 'atest'.
'cid' going inside 'atest' is 'ACID TEST'.
'giving' acts as a link.
(Other definitions for acid test that I've seen before include "Rigorous or crucial appraisal" , "Conclusive trial of success or value" , "Dictates to undergo conclusive trial" , "Conclusive moment (using litmus paper?)" , "Conclusive proof" .)