Cassette unravelled in groundbreaking action (4,4)
I believe the answer is:
test case
'groundbreaking action' is the definition.
(groundbreaking work may be done in a test case)
'cassette unravelled' is the wordplay.
'unravelled' is an anagram indicator.
'cassette' anagrammed gives 'TEST CASE'.
'in' is the link.
(Other definitions for test case that I've seen before include "One may set a precedent" , "Possible legal precedent" , "new suit?" , "Legal precedent setter" , "that could set a precedent" .)