Order dictates it's a critical assessment (4,4)
I believe the answer is:
acid test
'a critical assessment' is the definition.
(the most important test of something)
'order dictates' is the wordplay.
'order' is an anagram indicator.
'dictates' is an anagram of 'ACID TEST'.
'it's' is the link.
(Other definitions for acid test that I've seen before include "Validity confirmed" , "that one should run for gold?" , "Ultimate proof" , "Conclusive trial of success or value" , "Cat's diet (anag.)" .)