What does the cryptic crossword clue "Cassette unravelled in groundbreaking action (4,4)" mean?

The definition is "action". The wordplay uses an anagram. anagram of CASSETTE = TESTCASE; definition: "groundbreaking action" The answer is TESTCASE.

Cassette unravelled in groundbreaking action (4,4)

Guardian Cryptic #28286 | 14 Across | 2020-11-09 flower