The most common and most likely answer to this crossword clue is the 6 letter word MUCKLE
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More clues leading to the result MUCKLE
Buddy explains!

The straight part
We know that "to a large extent" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case to a large extent = muckle.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 MUCKLE mean?
MUCKLE noun- A great amount.
- To latch onto something with the mouth.
- To talk big; to exaggerate.
- (obsolete outside Northumbria and Scotland) Large, massive.
- (obsolete outside Northumbria and Scotland) Much.