The most common and most likely answer to this crossword clue is the 8 letter word SUPERMAX
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The straight part
We know that "extremely secure" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case extremely secure = supermax.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 SUPERMAX mean?
SUPERMAX noun- A prison with the highest possible level of security.
- Being, or pertaining to, such a prison.