The most common and most likely answer to this crossword clue is the 10 letter word THUNDERING
I've seen this crossword clue in The Times
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Buddy explains!

The straight part
We know that "Very great" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case very great = thundering.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 THUNDERING mean?
THUNDERING verb- To produce thunder; to sound, rattle, or roar, as a discharge of atmospheric electricity; often used impersonally.
- To make a noise like thunder.
- To talk with a loud, threatening voice.
- To say (something) with a loud, threatening voice.
- To produce something with incredible power
- Of, pertaining to, or accompanied by thunder.
- Producing a noise or effect like thunder; thunderous.
- Very great; extraordinary.
- A loud percussive sound, like thunder.
- A thunderstorm.