Cryptic crossword clue
In the main it constitutes a seismic convulsion 8

The most common and most likely answer to this crossword clue is the 8 letter word SEAQUAKE

I've seen this crossword clue in The Times
S
E
A
Q
U
A
K
E

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The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.
This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.

What does SEAQUAKE mean?

SEAQUAKE noun
  1. A hydrostatic pressure disturbance caused by an earthquake or volcano in the seabed.

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