What does the cryptic crossword clue "In the main, it's a source of upheaval (8)" mean?

The definition is "In the main, it’s a source of upheaval". The wordplay uses a cryptic definition. Cryptic definition The answer is SEAQUAKE.

In the main, it's a source of upheaval (8)

Times Cryptic #28760 | 3 Down | 2023-11-14 flower