Cryptic crossword clue
Bottomless den concealed by foxy to a great extent 6

The most common and most likely answer to this crossword clue is the 6 letter word WIDELY

I've seen this crossword clue in The Guardian
W
I
D
E
L
Y

Buddy explains!

The straight part
We know that "to a great extent" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case to a great extent = widely.
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 WIDELY mean?

WIDELY adverb
  1. In a wide manner; across a wide area.
  2. Commonly; generally; to a great degree.

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