Severe trial dictates changes (4,4)
I believe the answer is:
acid test
'severe trial' is the definition.
(I've seen this before)
'dictates changes' is the wordplay.
'changes' indicates anagramming the letters.
'dictates' with letters rearranged gives 'ACID TEST'.
(Other definitions for acid test that I've seen before include "Severe or conclusive examination" , "Severe trial; old assay method" , "Cat's diet (anag.)" , "Ultimate proof" , "Conclusive procedure" .)