Cassette mix is one that sets a precedent (4,4)
I believe the answer is:
test case
'a precedent' is the definition.
(I've seen this before)
'cassette mix' is the wordplay.
'mix' indicates anagramming the letters.
'cassette' anagrammed gives 'TEST-CASE'.
'is one that sets' is the link.
I am not very happy about this link. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for test case that I've seen before include "One may set a precedent" , "Legal precedent-setter" , "Cassette containing significant lawsuit" , "Legal precedent setter" , "Lawsuit whose decision will form precedent" .)