The most common and most likely answer to this crossword clue is the 8 letter word INSOMUCH
I've seen this crossword clue in Financial Times
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More clues leading to the result INSOMUCH
Buddy explains!

The straight part
We know that "to such an extent" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case to such an extent = insomuch.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 INSOMUCH mean?
INSOMUCH adverb- Because of; since; as a result of.
- To the extent that.