Bottomless den concealed by foxy to a great extent (6)
I believe the answer is:
widely
'to a great extent' is the definition.
(I've seen this in another clue)
'bottomless den concealed by foxy' is the wordplay.
'bottomless' means to remove the last letter (the letter at the bottom is removed).
'concealed by' means one lot of letters goes inside another.
'foxy' becomes 'wily' (synonyms).
'den' with its final letter removed is 'de'.
'de' inserted within 'wily' is 'WIDELY'.
(Other definitions for widely that I've seen before include "Extensively" , "over a large area" , "To a great degree" , "Generally" , "To a large extent" .)