The most common and most likely answer to this crossword clue is the 6 letter word FORBAD
I've seen this crossword clue in Financial Times
F
O
R
B
A
D
More clues leading to the result FORBAD
Buddy explains!

The straight part
We know that "Denied access" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case denied access = forbad.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 FORBAD mean?
FORBAD verb- To disallow; to proscribe.
- (ditransitive) To deny, exclude from, or warn off, by express command.
- To oppose, hinder, or prevent, as if by an effectual command.
- To accurse; to blast.
- To defy; to challenge.