Crucial trial dictates changes (4,4)
I believe the answer is:
acid test
'crucial trial' is the definition.
(I've seen this before)
'dictates changes' is the wordplay.
'changes' is an anagram indicator.
'dictates' with letters rearranged gives 'ACID TEST'.
(Other definitions for acid test that I've seen before include "crucial experiment" , "Cat's diet (anag.)" , "Rigorous or crucial appraisal" , "'Appraisal to show the true value (4,4)'" , "Dictates to undergo conclusive trial" .)